I saw IM3 in 3D today; while watching the closing credits I noticed that the post-conversion was supervised by Stereo-D, which did the excellent Jurassic Park conversion. As far as IM3's 3D goes, there was a decent sense of depth most of the time but the film was clearly not shot with 3D in mind - no real consideration of placement of objects and lots of quick cuts during the action - so it almost never really enhanced the visuals for me. Also, much of the film takes place at night or in dark areas, further flattening the image.
As for the film itself, I really was kind of disappointed by it....Warning: Spoiler! (Click to show)
Stark really doesn't even wear his armors for most of the film and when they're used they come across as incredibly fragile and quickly destroyed. The big action scene at the end was also a letdown as all the armors seen in the previews make appearances but nearly none of their specific functions come into play as they just fly around taking random potshots. Rhodes/War Machine/Iron Patriot is practically an afterthought and the way in which Ben Kingsley is used was literally laughable and kind of off-putting given how the trailers and clips led you on. Killian (Guy Pearce's character) is a literal copy of The Riddler from Batman Forever, right down to the nerdy first appearance and rejection by his hero. Even the post-credits scene, a staple of these films, is somewhat amusing but largely superfluous and not used to hint at what might come next, as was often done in these films.
...so overall it was easily the weakest of the three Iron Man films for me.