First of all, whatever I say now is subject to change, because I really don’t think the show is done with Faith (when a story puts someone in a coma instead of actually killing them it’s always for a reason, no matter how loudly they tell us “Oh, it’s ALMOST IMPOSSIBLE that they will ever wake up!”)
The short answer is that I really liked Faith and felt horrible for her by the end of season 3. She made some pretty terrible choices, but what I liked about her portrayal is that none of those choices were shown to be inevitable. I don’t mean this in a ruthless “Oh, she had the choice to do the right thing but didn’t, so she really really sucks” sort of way. It’s just that an unfortunate combination of events pushed her over the edge, but the story allows us to see the moments where things could have gone differently, if only she had opened up to her friends; if someone had said something different; if she’d allowed herself to believe that people cared enough about her not to give up on her. There was nothing intrinsically horribly about her, I don’t think; she was just lonely and lost and pretty terrified. She lacked the support system Buffy had and used to keep herself grounded, and that only makes her story sadder.
Something I definitely did not like about her portrayal, though, was the way her sexual forwardness was used by the narrative to signal “badness”, or at least the potential for it, in a very traditional “sexy villainess up to no good” sort of way. This goes hand in hand with what I was saying in a comment yesterday: the show is really ambivalent about its female characters’ sexual agency. On the one hand it puts them in control of their sexuality, but on the other hand, it often punishes them horribly for it. And although it allows them to be both sexy and smart and resourceful (like Buffy), it also doesn’t like them to be too sexy. The fact that Buffy is a bit demure in comparison to Faith is supposed to tell us she’s a better person, which is pretty crappy.