She was okay as Buffy. The film wasn't as good as it could have been, but she did fine with the role that was lighter and sillier.
Sarah was obviously the better Buffy. Not only did she get better material, but I think she suited the role better as she was more vulnerable and had a slighter build which you need for the character who is kind of like a Spider-Man type, IE deceptively weak looking.
Still credit to Kristy she introduced the role. I know that she is friends with a lot of the cast from the tv show. There is a great panel of her, James Marsters, Amber Benson, Mark Metcalf and Claire Kramer all together on youtube, and there is a famous one of her with Charisma and Julie Benz (I think that was when Charisma first mentioned what a douchebag Joss was to her on Angel LOL.)