I like her as an actress. Now obviously, I'm a bit biased because of Buffy, but I've found that even when I don't enjoy the movie she's in, I like her in it, if that makes any sense.
"I know what you did last summer" is one of my all time favorite horror movies, and I actually saw it before I'd even heard of Buffy, and I liked her a lot in that. Sure, it was a pretty one-dimensional role looking back, but it's still a fun movie.
Ringer was a show I thought was decent, but I enjoyed SMG in it, and she was the reason I kept coming back.
To tell you the truth I've never seen The Grudge movies, and I couldn't stand Scoobie-Do (seriously, I have my own little grudge against those movies. They're the reason there was no Buffy in Angel S5) or the sequel. I liked her in Cruel Intentions a lot though (and there is one scene in particular 😀).
My only real criticism is that I'd like to see her in a bit more. I understand she's right in the middle of family life now, but I enjoy watching her, so purely as a fan I'd like to see more of her work.
But of course, Buffy is where she's the best. She took a role that could have wound up being a one season bomb and turned it into one of the most iconic and enduring characters of all time. Not just anyone could have done that. She's a remarkable leading woman.