
The power to cleanse, the power to destroy — these are the things that fire and water have in common, and in this episode we saw quite a bit of both.
The flashback storyline is a mirror image of Charlie's dilemma on the island. The only difference is the Pace brothers switch roles. In either storyline you've got someone struggling to put his demons to rest while trying to ensure the safety of a baby. In both cases they're kicked to the proverbial curb by the baby's mother.
Charlie's keyed up by lucid dreams he's been having about Aaron, which unfortunately causes him to overreact to Eko's suggestion of baptism. Thus Charlie's quest becomes clear — cleanse the baby through a sacred ritual.
So far so good... so why is this episode so polarizing in the LOST community? I suspect it might have something to do with the "destroy" part of the equation. That is to say, this episode comes very close to destroying the Charlie character beyond repair. It wasn't so much that the Losties were universally against him — we've seen that the majority can indeed be wrong.
No, this time it was different. We witnessed Charlie lying to multiple people repeatedly while looking them square in the eyes. Therefore we could not trust the narrative presented by Kitsis and Horowitz; just because we didn't SEE him using heroin didn't mean that he hadn't. We wanted to believe that he was simply having bizarre dreams, that he was sleepwalking. But after so many blatant lies there's no way to know.
All that said, I don't think any of this qualifies as horrible writing and I'm still unclear on why so many crucify (pun intended) the episode. It's not the first time we've seen characters take considerable damage to their reputations, nor is it the first time we've witnessed strange visions. What makes F + W so different?
On an unrelated note, a slight deduction is in order for what seems to be a goof in the dialogue between Hurley and Libby. Libby says she remembers him from the Oceanic flight, that he was the last one on the plane and was sweaty and he stepped on her foot. She was in the tail section and he wasn't; how could he have stepped on her foot if he was the last one on?

Say what you want about Fire + Water, Dominic Monaghan was a good sport while his character was systematically obliterated. He was certainly convincing as a lunatic who'd broken one too many statues in his day. That said, I always cringe at the flashback scene where he shares his new song with Liam. The substandard singing voice is bad enough but then he does this ridiculous, inexplicable head-rocking thing between verses.
Emilie de Ravin finally gets some of the spotlight in Season 2! She plays "emotionally wounded" very well and it's a nice change of pace from bloodcurdling screams in the caves.
Terry O'Quinn certainly showed a level of feistiness in F + W that we'd rarely seen before. I guess it's pretty clear that you don't mess with anyone Locke has committed himself to protecting. It's not my favorite type of role for him, but that's not to say he's not extremely good at it.
I wasn't too impressed with any of the guest cast, though I will say Neil Hopkins deserves credit for the scene where he confesses to dropping Megan.

Okay, this one's a bit tough. How do you rate visuals and effects when the dream sequences are intentionally bizarre? I guess you do it by rating them strictly on visual effectiveness and setting aside opinions on their propriety and relevance.
Some of the dream sequences were nicely executed, particularly the piano in the ocean and the Christmas Morning sequence. The vision of Claire and Charlie's mother — eh, sorry, it looks as silly on-screen as it does in the script.
Even though the Butties commercial feels like a heavy-handed attempt at a laugh, I have to credit it for being effective from a visual standpoint. Boy, I just sounded like I have no sense of humor, didn't I?

I don't have the seething rage toward this episode that so many others seem to have, so I'm not repulsed at the idea of watching the episode repeatedly. That's not to say I'm a big fan of watching Charlie get destroyed before my very eyes, though.
Maybe some people are really turned off by the religious overtones, thus their Longevity rating would suffer. I've personally embraced the spiritual themes throughout the series so I'm not opposed to watching it time and again based on that alone.

Atmosphere is where I think we have a problem. To put it bluntly, Fire + Water has an identity crisis and that falls on the writers' shoulders. Kitsis and Horowitz are very good at writing humorous episodes and that actually works against them this time around.
Whether you subscribe to any particular religious beliefs or not, it's generally understood that a baby's health and the idea of spiritual cleansing aren't exactly "Night At the Apollo" material. F + W would've been far better served if it'd taken a more serious angle instead of forcing in humorous bits like the Butties commercial.