Despite the high expectations going in, I still felt it was a good movie. The plot and character development were surprisingly sohpisticated. Much more like the comic books I remember from years ago than the superhero movies of the last decade or so.
The joker was much more like my memory of him than the Tim Burton joker. Having said that, I think that Heath Ledger was a bit flat and is getting a lot of credit that should have gone to the script writers.
As with many films in this genre, I felt that the fight/battle sequences were too long, and there more to exploit FX technology (and the incredible sound system at the IMAX) than to support the story.
The IMAX experience was very good. Next time I will sit a little farther back though, the screen was a little to large to completely capture in my field of vision, so I felt like I was watching a pan-n-scan at moments.