Yeah, the effects were cool, but the story line was oh so predictable. Also, I thought some of the acting was pretty mediocre (although Sam Worthington as the marine and Sigourney Weaver as the scientist were both good - Sigourney very good).
I'd give it 2 and a half stars out of four. My wife and two teenaged sons both thought it was better than that.
For anyone further interested, here's the review I put up on my own blog -
http://tomfaranda.typepad.com/folly/2009/12/avatar-3d-my-take.html