Got my hands on a copy and got a chance to watch through the last half hour or so. I chose to check out the 3D version... haven't looked at the 2D yet. I also watched it in 2D in the theater as I'm not a fan of conversions, so I can't compare to the 3D at the theater. Technically speaking the 3D was well done. As all conversions are, at least from my experience, a lot of scenes still have that muti-tiered storybook look to them. Like cut-outs on a canvas. I know a lot of people here liked the Captain America 3D conversion and similar, but it wasn't my cup of tea... looks unnatural to me. This was a level better, but it's not fooling anyone of being a conversion. It just lacks the same nuance and levels of depth compared to something like Avatar or Hugo. However, there are so many fast moving action scenes that once the action ramps up, you don't have enough time to notice that effect and can just enjoy the added depth to the action. In other words, when things are blowing up I liked the 3D a lot and it added to the experience. As well done as I could expect a conversion to be. As far as the quality of the 3D itself (conversion or not) it was well done, easy to watch without any ghosting or issues. We have a great setup with a bright Titan and RealD glasses so I almost never see ghosting unless it's really a technical issue with the movie itself. This one looked great to me on a large screen sitting about 10ft away. Up close and personal as 3D should be watched. ; )
The audio was excellent as well as others reported. I wouldn't say nuance is the name of the game with this one. Not like MI:4 where it's just masterfully done in every scene. That audio is so good it almost calls attention to itself and takes you out of the movie. Avengers is still easily a 9 out of 10 with a lot of loud, powerful bass. I don't recall any specific scene standing out with super low bass... as an oh my god that was deep I have to go back and play that again (like moments in War of the Worlds, Live Free, Hulk, etc). I'm sure other guys here will run their software and post the freq. extension. But it was clean, powerful and a joy to listen to.
I still need to watch it from start to finish, 2D and 3D, but in the meantime just wanted to post some thoughts while I had a moment. Besides wishing the film was in 2:40 so I could open up the whole screen, I really have nothing to complain about. Solid job and looking forward to seeing how good the 2D looks.