I told everyone this would be a surprise. The question is, why review this one? There are two reasons. First, there isn't an episode other than "Ghostfacers" more polarizing among fans, thus that makes it worthy of a critical review.   Second, I felt like it.
"Heart" instantly commanded attention since it combined the writing of Sera Gamble with the direction of Kim Manners, which usually guarantees a memorable episode to come. However, many fans didn't like it because they thought it went overboard with the Sam strife and broke out all the anvils. Was the overwrought emotion in this one heavy handed? Yes. Was it compelling to watch? Yes. 
Upon deeper examination, I found "Heart" is far more complex that what appears on the surface. The episode isn't just about Sam. Dean is forced to face some realities as well, and in the end, it hits them both hard. Sure, it took an implausible setup to do it, but whatever, drama is drama. 
The Recap
In an unusually long intro, we meet Madison, who has an ass for a boss, and a stalker. The only part of this intro I liked is the setting of San Francisco, which is pretty cosmopolitan for this show. I had my honeymoon in the Bay Area, so I have a certain fondness for the area. Madison finds the boss in the office next morning looking like a spit up hamburger, and somehow, I'm not feeling shocked here.  He looked like dog food to me from the first appearance.
Sam's in the morgue investigating and I'm wondering how he got this detail, since he's the brother with the weaker stomach. He must have lost a coin flip, since we know he never ends up on the losing end of rock, paper, scissors. He goes back to Dean, whose cleaning the guns. Is that all Dean does in his off time? I have a neighbor who's constantly washing his Cadillac, like every other day, so I wonder if there's a little OCD going on there. I'm worried about Dean.
Ah, but now Dean gets all excited, for he gets to off a werewolf, and has to silver bullets needed. This time Sam gets in the smart ass line, and could you picture the Winchesters at Disneyland? Yeah, that scares me more than werewolves. They talk to Madison, who Dean immediately makes nice with, and she mentions the evil ex-boyfriend. What a weak red herring. 
The brothers break into Kurt's place, and he collects model cars. That right there tells me he's no evil werewolf. Plus we know that it's never the first suspect, otherwise these shows would be twenty minutes. A cop becomes this week's red shirt, and Sam and Dean race in just to time to catch the fresh hamburger mess number two. A bit late there guys?
Using a logical (?) thought process (we know what they were thinking with), the brothers check on Madison, because she could be werewolf dinner, and she's hot.  Dean suggests someone stays to watch her, like himself since he's the one that gets all the girls, but surprise, Sam wants to stay. Sam does the one thing he knows can beat Dean, and we get a HUGE treat, the brothers dueling it out in rock, paper, scissors, a game Dean apparently sucks at. Oh Sammy, you like her!  Strange how he won't come out and say it. I'm not sure if anyone else found this remarkable, but outwitting Dean at a chance to be with a girl is so not Sam. 
This part raised such a stink, and I'm not sure why since it's an innocent, playful moment. Madison folds her underwear in front of Sam, making him uncomfortable. It's meant to show how carefree she is, as opposed to Sam's uptight behavior. Opposites attract. Plus Sam's reaction was funny. Dean even adds to the moment, calling Sam and pushing him along, but he isn't there to poke him with a stick, so Madison chooses to do it.  Sam won't make a move unless some hot chick whacks on the head with a sledgehammer.  Their time on the couch watching soaps is endearing, and seeing Sam finally lighten up and shyly compliment Madison while sweetly showing his attraction is a refreshing change for his character. Up until now, it's been 39 episodes of Sam moping. 
In the meantime, Dean gets to chase the waste of time, and it's "Down The Street" by the Stooges! What an obscure yet really cool song to drag out of the archives. There's an attack, Dean charges in, and unless Madison has a twin, Sam's got some explaining to do over his keep an eye on her. Dean cleverly nicks her with the knife, thus proving the next morning there's no such twin. 
From this point forward, Sam goes mega emo, and I see a striking resemblance to him from "Mystery Spot". Time to ride the emotional rollercoaster. Jared breaks out the sad puppy dog eyes and quivering pout, and now the fandom is split between wanting to give him a hug and wishing he would man up.   Put me in the hug category. 
Madison's a killer, and Sam makes the black and white judgment, but that doesn't last long. She appeals to his inner demons, and he wants to believe she has a monster inside that she can't control, because he's convinced he's the same way. He has to save her, otherwise, there's no hope for him. Yes, Sam has some baggage.   
Dean reluctantly agrees, probably because he sees how riled Sam is, and goes out to where all the attacks happen, while Sam stays with Madison, promising Dean in unhinged fashion that he will kill Madison if he has to. While waiting, Sam looks out the window, anxiously fixed on the full moon rising and we wonder if Sam is thinking about Madison or himself. The full moon has been a constant symbol throughout this episode, so I dug for any hidden meanings. All I could come up with was something simple; the moon has a bright side and a dark side.  The full moon is beautiful and creepy at the same time, for it reveals two sides to every soul. It represents good and evil. Yeah, I might be fishing a bit with that one, but it makes sense to me. 
Of course the moon also means that one very hulked out werewolf Madison gets to slash Sam in the face, but Sam recovers from his carelessness and gets Madison in the closet. He moves the heavy entertainment center with TV in front of the door, and I'm asking how he can do that but not take on a werewolf? 
Next we get a very common trick from both Kim Manners and Sera Gamble, push the emotional threshold beyond its limits.  Dean shoots werewolf Glen, the neighbor who turned Madison, who dies in front of Dean with no idea what's going on. Dean's adrenaline drops to outright compassion, and thanks the Jensen's unbelievable acting, we experience every gut wrenching bit of it. This was another crushing blow to Dean's black and white world, for even monsters are victims too. For such a simple moment, it took us to a place we never expected, pure sympathy for the villain. 
Then Jared gets his turn, and it's just as powerful. The somber music plays as a glum Sam opens the closet door, revealing a normal Madison and the intense damage within. He sadly delivers the line "You'll never see me again" and disappears. At first, I didn't get why that was so emotional, aside from the heartbreak on Sam's face killing me. After some pondering though, I got it. Instead of getting to know this incredible woman who he's deeply attracted to, he has to leave her, and as he said earlier, "I'll just be a bad memory." Sam's facing his dark side, and even though what he did to her was right, it doesn't sit well with him. He wishes he could be someone else. Hmm, he's wished that his whole life, hasn't he? See, I told you there was more to this episode than what was on the surface. 
Sam and Dean hang out in front of Madison's house in the Impala, and I'm thinking it's about time someone called them out on how obviously they stick out. The Impala is not a conspicuous car!  They wait out the evening together in the apartment, all watching that vivid moonrise together, but Sam and Madison end up spending all evening enamored with one another on the couch, while Dean watches the moon alone. Poor guy, he looks bored. I'm sure there are plenty of volunteers to keep him company. 
Dean ends up giving Sam a bit more than a gentle nudge. When all is clear, he announces he's leaving to watch pay-per-view. Big hint number one. Even Madison got hint number two, the hilarious fist pump in the air as he was leaving. Dean isn't known for being subtle, is he? Of course Sam is all freaked out and stammering with some crappy excuse, but he caves quickly when she jumps on him, spinning her around and slamming her against the wall in a fit of passion. Funny how it takes two overly forward people to get Sam laid.
Now Kim Manners is in full control, because Sam and Madison certainly aren't exercising any given that their clothes are off and they're mauling each other in all sorts of ways. He opens up the director's handbook and dusts off the chapter, "Keys to a Great Sex Scene." Step one, pick the right song. Check. The Screaming Trees' "Look At You" is quite haunting. Not only does the song set the perfect mood, but the lyrics are so incredibly appropriate. Look at the chorus alone. "When I look at you I've got a second chance, really need to have you now, one by one they fall, it always breaks me down."

Wow. Okay, step number two, get two people that look really hot naked, and use every trick in the book to accent the hot and sweaty. Check. Step three, make the sex steamy, passionate, and well choreographed. Whoa, is Sammy actually taking her from behind? Hee hee, our horndog is a back door man. Check! Step four, the sweet, intimate final shot, one that melts our heart with both characters content on the bed in each other's arms, sleeping the best they have in years, and looking very sexy doing it. Textbook, and it's great.   
Now the moon again and holy crap, how long did they actually do it? They jumped each other shortly after the sunrise, so they went all day long? How much pay-per-view is poor Dean watching by himself? The bliss is short lived for Madison turns, and what a better way to go out on a mauling than by swimming in her giant lover's oversized shirt. She wouldn't want to wear anything skimpy for that.
Sam rushes to Dean, wearing only his grey undershirt under his jacket, and I'm thinking he should change his look, since all this dressing in layers is too inhibiting. We get some fine detail from that shirt. Sam is understandably emotional and won't listen to Dean's attempts at rationality. In his outburst he even berates Dean on the fact that he can't kill him, even though he's evil, but he can kill Madison with no problem. Whoa, going for the jugular there Sammy? You're his brother, of course he can't.  
Madison calls Sam, shirt in tact, and now for the part that throws everyone in a tizzy. Back at the apartment, Sam won't give up, but Dean and Madison are a little more practical. I'll admit, Madison asking Sam to kill her was a heavy handed plot turn. Granted, she doesn't know Sam's sad history, but come on, you just spent an entire day rolling in the sheets with this guy, getting to know him in every way imaginable, and now you expect him to off you? Cut him some slack. 
But no, we need the setup for the big emotional finish, so I suspend believability and watch it play out. One of the major criticisms I heard was Jared overacted, for there was no way Sam would be that upset over killing a girl he recently met. That would be true except that his upset had little to do with Madison. This is the Sam breakdown we'd been waiting for all season. All those fears, anxieties over his destiny, over the monster inside of him, over John's warning to Dean, well, he clearly hasn't dealt with it properly. He's been holding it all in, giving us only glimpses of his worry in small moments like getting drunk in "Playthings" and his loss of faith in "Houses of the Holy". Both times his fears were swept under the rug, continuing to fester and grow inside. He finally meets someone he opens up to, whose just like him, carrying an uncontrollable monster inside. When she couldn't be saved and admits defeat, Sam's world crashes down on him. There's no chance for him either. So yes, emotionally he lost it, big time.
But, notice how the final shot didn't end with Sam. It ended with Dean. There was a huge reason for that. The person ultimately responsible for saving Sam is Dean. He watches Sam crumble, all hope lost in his brother's eyes, and for one brief moment there, he doubts that Sam can be saved. He finally realizes how much everything has taken its toll on Sam, and how much he's not alright. 
The lone tear down the cheek, cliche as it might be for Dean, was also very genuine. We saw Dean picturing in his mind the exact moment when he would have to do the same to Sam, and the cringe from the bullet firing made his living that moment all too real. One reason Sam accepted the task of killing Madison was to prove a point to Dean, that despite how difficult and devastating the duty is, killing something evil, no matter what they mean to you, is what must be done. Dean got the message, and he didn't take it well.
Whew! It's a good thing "Hollywood Babylon" came next, or it would have been the funny farm for not only Sam and Dean, but all of us. I'm not going to grade this one, probably because it's really hard to come up with a standard based on the huge amount of emotional baggage. The episode served its purpose, and we went on. Next time, I close out season three, commenting on the new DVD set, and what we learned from the season in general. I'm ready to move on; bring on season four!