Whatever Happened to SMG?

I was a huge fan of Buffy in the 90s and though the star would go on to big things. I caught one of her movies, Simply Irresistible, and wondered if her success might not have more to do with Joss Whedon than natural talent. It seems that many of the actresses--except perhaps Alyson Hannigan--he works with seem to thrive in his series but wilt without him.