Personally, I'd much rather see her on a weekly basis than once in a while on the big screen. I really think her acting is, how should I put this, mediocre? The funny thing is, she's probably one of my top 5 favorite actresses! And that's just because I used to watch her when she was on All My Children and then followed her to Buffy.
Unfortunately, she's the type of actress that needs a good length of time to establish her characters. I just believe that she is more suited for tv, and I believe that she could get out of the Buffy typecast if she took on another television role. I mean, look at Christina Applegate. She had one of the hardest typecasts to shake, and by golly the girl has done it. Granted it took a few mediocre film roles, The Pussycat Dolls and Broadway before she took on Samantha Who? but she's pretty much reinvented herself. She's not referred to as Kelly Bundy anymore, but Christina Applegate.
Also, look what tv did for Patrick Dempsey's career, he wasn't a largely known actor at all, and then Grey's Anatomy came along and now he's everywhere. Though he's now stuck in the McDreamy typecast...
SMG is going to be known as Buffy until she makes the right choices to get out of that typecast, and I don't think these roles she's played the last 5 years are it.