People have described how they find her appealing because she continues to do this in the face of everyone telling her she's not good enough.
Yes, this. And I think a lot of the differing opinions about Steph are based around this perception, and what it implies.
Basically, those of us who are defending Steph are seeing her as the hero, the center of a story. As such, we have expectations of the way the story should treat her--as someone with potential, as someone worthy of something, as someone who can go the distance. We see her being impulsive, argumentative, and so on, and to us those flaws are things she needs to grow past (or just automatically metamorphose into the Hal Jordan hero archetype, where those flaws are presented as virtues, but I hate that archetype, so not so much for me) so that she can grow into the potential we believe she has, because she's a hero and should be in a hero story.
On the other hand, the official viewpoint most if not all of the time has her spend pretty much all her page time up to now as an adjunct to Tim's story. As such, she's there to show how not everyone can be as good as he is, mess up so he can be thrillingly heroic, and generally serve as one of the kids-don't-try-this-at-home stories. And I hate those stories. I hate with a burning passion the stories that say, "This character is Special and Different, and this other person is Not, so the other person can never be as good." HATE. Mostly because I'm always more inclined to put myself in the place of the other person. I identify with Steph more than Tim.
Besides, I find it highly problematic that it's a working-class girl who's Just Not Good Enough compared with a rich boy. I know that's not why she's presented as Not Good Enough, but it's still a problem for me both intellectually and emotionally. Intellectually, because it reinforces assorted -isms, all of which are bad. Emotionally, because I identify with the cheerful, impulsive, thinks she knows better even when she doesn't and hates admitting it girl, and "you're just not good enough, no matter how much you want it," is just not a story I'm willing to accept.
I think the difference of opinion comes from the fact that we're reading different stories. Apparently "impulsive girl screws up, fails, refuses to admit it, should just go home" is a story you're fine with, or a story you're not focused on. I'm not fine with it. I want a story where "impulsive girl tries hard, fails, tries again, succeeds, shows all the people who said she'd never be good for anything". And since no one seems inclined to actually give me that story in comics, I'm perfectly willing to take it.