I always found Cordelia's character development very believable. Can even (if I grit my teeth and try to ignore all the stuff I hated that they did with her in season 4) see what happened to her with Jasmine as a result of a touch of hubris on her part, just as Skip said. Why on earth should she think she'd qualified to become a higher power etc?

On the other hand, I agree entirely with Stormwreath. People who inveigh against the changes in her character in season 3 are either crazy Bangel 'shippers, who are invested in having her not change because then Angel couldn't possibly fall in love with her or people who wanted the Scoobies to stay at High School forever and never change.

Eh, dear! Hadn't realised I was still so bitter.