I saw Avatar 3-D last night... got back home around 2am finally fell asleep around 3am. What a great film. I think they are right to call it groundbreaking. Sure...it gets a little preachy and the story is borrowed from several other films, but the way Cameron creates Pandora and the cinematography make it truly enjoyable. It didn't really lag for me at all and I was dog tired before the film even started.
The 3-D effects were integrated so well that they weren't announcing themselves every two minutes but really added to the story, particularly the aerial scenes. The close-ups of the lead actors were really hard to distinguish CGI from real... it is only when the "warpaint" gets introduced that it starts to look a bit cartooney. Anyway, it is easily up there in my top 10 favorite movies, along with Indiana Jones (the first one), Star Wars (the first one...now the 4th one 
), How The West Was Won ... If you haven't seen it ... go see it! It is worth the price of a ticket and then some IMHO. ;-)