I saw it at an IMAX that a local theater had recently installed, and although I got bad seats in there because my brother took us late, I was really blown away. I was surprised how packed the theater was considering we saw it at 3:25pm on a Monday, and there was an incredibly long line outside the theater once we got out. Maybe they were waiting for other movies too, but my guess is that they were also waiting to see Avatar.
The thing I loved about the movie was the little details. For example, when he first enters the body, and starts running around in it, you could see the dirt moving around his feet, something that's hard to accomplish easily in normal CGI-only movies (which is why their worlds often seem sterile). Or when a fight occurs, and things get thrown around, you could see dirt/leaves/grass moving around as well.
I loved how cool the human base looked on the inside. The 3D was not overdone, it really helped to make it look like I was inside the base with them, watching it from a few feet away. I bet we'll also have computer images and displays like that within 30 years or so.
Not that this alone would sell the movie, but it really enhances it and makes it seem like a world that's existed all this time, not suddenly sprung out of existence out of nowhere. I remember reading somewhere that Cameron was influenced with this because of Star Wars, and how it seemed like a lived-in universe unlike earlier sci-fi movies where everything was crisp, clean, and brand new.
The 3D wasn't perfect all the time though. Sometimes it got really distorted and really broke up. So the image that the camera was focused on was crisp, but then everything around it was terribly out of focus and wasn't even recognizable. Such as
the scene where the Naavi were burying the dead one. Everything close up was recognizable, but the background stuff was so blurry as to make you wonder if your vision was distorted or something. But it only lasts for a few seconds, so it doesn't break you out of it.
One minor nitpick I have is when Michelle Rodriguez is flying the helicopter. How come she never wears the mask when flying? You can argue that they're in a sealed environment...but you can see this in the trailer where she says "
ou should see your faces." The problem with this is that the back door to the cockpit is open, meaning she's breathing in that poisonous to human air. Nothing that's going to ruin the movie, but still considering the amount of attention put into the other scenes, you'd think Cameron would've noticed this.
One other thing I wished the movie had more of, mech vs. naavi combat. The last fight scene was pretty awesome, but it would've been great to see at least one other fight like that, considering they were about the same height and what not.
Personally, if I were in charge, I'd just nuke them from orbit, just to be safe
But then there'd be no movie either, so I can see why Cameron did this. Will be interesting to see what happens in the sequels now.
R.I.P. Tom, You will be missed 
erhaps bland isn't the word I'm looking for