I have complicated feelings about Faith.
I don't agree that she was a "good girl first then went bad" or that "she wanted to do good originally, then turned bad". On a superficial level she seemed that way, but after scrutinizing her a bit I came to the conclusion that, from the moment she arrived on the show, she was more into herself and pleasure than anything else. She wasn't originally performing slayer duties because she appreciated the good it did. She did it because she got off on it. She would say all the time that killing vamps "got her juiced" and she tells Buffy things like "Tell me you don't get off on this!" and "Slaying is what we were built for. If you're not enjoying it, you're doing something wrong." She never said anything about feeling good that killing vamps was doing a good thing for society or innocent and helpless people. It was really more about the pleasure of it all than anything else. And it was this feature about her that convinced me that she was really never good in the first place. I don't consider someone good if they are a slayer simply for the pleasure (and at bottom that's what it was about for Faith in the beginning, pre-reform). I consider that person a simple pleasure-seeker, a hedonist if you prefer.
Add on top of that that she is always (especially in the beginning) choosing fun and play over everything else, including slayer responsibilities sometimes, and I had a strong reaction of dislike towards her originally. She was all about dancing with an endless line of random guys at the Bronze and other places, she was always thinking about sex (she would say things like "God, I could eat a horse. Isn't crazy how slayin' just always makes you hungry and horny?") and hitting on every guy she met (including Giles, if you remember, the first time they met too). I don't know.......I just had a strong impression of her in those early days that she eschewed responsibilities for a life of carefree fun and gratification, and that she did slaying for all of the wrong reasons really (she got off on it). When, early on, Buffy's friends took to Faith I felt bad for Buffy. Sure, Faith upon first meeting seemed like the kind of person we would all like to be around, and she appealed to everyone's desire to have more fun, but Buffy was unquestionably the better person, if you ask me. She might have been less fun sometimes (because she appreciated the responsibility of her calling and took it seriously, more or less) but she was a better person by my analysis. Faith was more fun, but ultimately she did everything for herself, she was basically selfish: she was selfish because she didn't like serious responsibilities (because it detracted from her care-free and fun-searching lifestyle) and she was selfish because the reasons she performed and enjoyed slaying at all was because she got off on it, because it "got her juiced"; in other words, she did it for personal gratification/reasons and not for any selfless reason, like because it was the right thing to do. She never once said slaying was good for anything else, not once, even when Buffy would chime in that they were doing the world some good.
So yeah, how did I feel about Faith early on? I didn't like her, straight up. I didn't like her at all. And I never really thought of her as truly good either. Which made the whole "Faith was good and then fell from grace" kind of nonsense to me.
And yeah, I understand she had a horrible past and childhood, a horrible father, etc. But I'm evaluating her and providing my impression of her from the first time we actually meet her on camera. There were obviously reasons for why she was bad, but let's be clear, she was bad (in the beginning). Or at least, not good (in the beginning). Maybe bad isn't quite the right word to use either...maybe base would be a better word. Faith was base. Her way and attitude and mentality was base, vulgar, whatever you want to call it.
Yeah, I got to say I didn't like her at first. And she reminded me of similar selfish pleasure-seeking people I'd met in real life too.
After coming to terms with her bad (or rather unhealthy) self and attitude, I definitely liked her a lot more. She realized her attitude and lifestyle was a bad one during the body-swapping episodes. You could see during her conversations with Buffy's mom (while she was in Buffy's body) and with other people she was getting defensive about herself when people would treat her (while in Buffy's body) with the love and respect that the real Buffy always received and deserved. She could feel the goodness that a respectable and responsible life well-lived earned, and she got defensive. You could tell that Faith was having to finally confront the fact that she made the wrong choices in life, that she lived life selfishly, etc. The Faith/Buffy body-swapping episodes were brilliant. It was a brilliant way to expose to Faith the inferiority of her lifestyle and selfish attitude by being forced to live in the skin of Buffy, who was everything that she wasn't. Of course, Buffy isn't a perfect person, but that wasn't the point. Faith could finally step outside of herself and see herself objectively for what she really was, she could see (by way of comparison with Buffy, with whom she could experience from the inside out and see all of the love and gratitude she received for the lifestyle and attitude she had chosen) the errors of her way. The final blow came after having sex with Riley (as Buffy). Riley didn't want to use Faith (as Buffy) for sex, Riley loved Buffy. He made love to Faith (as Buffy). That was something that she had never experienced before, and it shook her. She wasn't the kind of person that at that point was capable of love (or being loved). Faith had always looked at sex as mere gratification, and she enjoyed it in a purely selfish way, her needs being the only priority, and so forth (see her one night stand treatment of Xander for one example). But Riley showed her what it meant for someone (through sex) to actually care for someone else in a meaningful way. Experiencing actual real love helped crack Faith even more, to the breaking point.
And then the final confrontation between Buffy/Faith happens and Faith loses it, beating Buffy (in Faith's body) and while she was on top of her and beating her she said things like "you're nothing, you're disgusting" over and over again. Faith finally broke: she wasn't calling Buffy that, she was calling herself that (literally beating her own face up).
And then she breaks down further on AtS with Angel and she hits bottom...and the rest is history. She climbs herself out and makes significant changes. At that point, I liked Faith. I felt sorry for Faith before all of this (in the beginning) but I didn't like her. Not until after she made some...realizations about herself. I liked her a lot in Angel S4/Buffy S7. In a way, a few of her features remained the same, but obviously a lot more had changed for the better. I haven't read the comics yet but I like Faith at this point.
Sorry, I am rambling on way more than I should, but I got started and so I felt I had to go ahead and finish it.
So yeah, to summarize, I didn't like Faith much early on but obviously as she changed for the better I found myself liking her more and more. But I definitely didn't like her in the beginning. At most, I felt sorry for her a little.