Now I've just seen this thread so it's maybe too late to comment, but I've just watch this movie on IMAX 3D and on regular 2D screening.
I won a competition for the IMAX so I took my Dad. I loved the size of the screen and I felt totally immersed with it. The 3D was OK but it's not a movie that was filmed in 3D so there isn't much happening in it. It was a bright picture and I could see well even on the dark shots.
In comparison, I went with a couple of mates to watch the 2D version and the reason was because one of my mates is a professional cameraman and he reckoned that it's a better experience to watch the 2D as it was intended. For me though, the contrast was pretty poor and the black were grey with little definition. Towards the end of the movie when the shots are bright it was OK. The screen was small (normal cinema screen size) so I was constantly aware of my surroundings, meaning every time someone got up talked I was aware of it.
Both had great seats - we booked VIP for the normal viewing and the IMAX were all 'VIP' seats. The viewing angle on the IMAX was steep so the people in front were nowhere in your field of view, which wasn't a good on the normal viewing.
I'd prefere to see a dedicated 3D movie in future on the IMAX, otherwise I'd be happy to just watch a standard movie but on the IMAX screen.
I'd give the IMAX 8/10 and the 2D version 5/10