If you haven't seen The Avengers (aka Avengers Assemble) yet. Run, don't walk, to the nearest theater and see it.
You won't gain much by watching it in 3D, but I would love to have seen it in IMAX. The Helicarrier scenes (most of the film) and the final battle scene all look like they were really meant to be seen in IMAX.
The only aspect that took me out of the film was that they had Scarlett Johansson's best assets featured on the screen every chance they could. I'm not complaining, but it pulled me out of the story a couple of times.
Mark Ruffalo, as much as I don't really like him, brought his A Game and killed it as Bruce Banner. I am actually looking forward to him playing Banner in further Avengers and Hulk films. Really, really good job.
It was Robert Downey, Jr., as Tony Stark/Iron Man who "stole the show", except he didn't have to. It was his show to own.
It is a little long at 2:20, so be sure to hit the restroom before the previews. And it is a Marvel movie, so you will need to sit through the credits. There are two extra scenes. The first is after the flashy, pretty credits and the second is after the complete, boring credits. The second one doesn't add much to the story or continuity...and yet it says a lot about what was going on.
My wife, my 8 year old daughter, my 5 year old son, and I all loved it for a variety of reasons.