Also saw it at the premiere on Thursday (it was technically embargoed till today, but they forgot to tell the people at the premiere that, so a bunch of reviews slipped out over the weekend). The cgi is astoundingly good, to the point you forget your watching cgi characters (which is something i never really thought i'd say, as i've always had issues with the 'dead eyed' look of cgi characters in the past.)
The story is a bit predictable, which is going to cause some negative feedback i think, but it's a perfectly serviceable plot. Plus you tend to stop thinking about the fact you know where it's going, and get wrapped up in how the film gets there.
I think Simon Pegg is right, despite what you think of Avatar overall as a movie, it is 'game changing' in that it shows what can now be done with digital characters and environments. The level of detail is quite breath taking. For that alone it's worth seeing. And I would see it at a cinema or IMAX, in 3D. It's the first movie i've seen where I thought the 3D actually made a real difference to the experience.