Okay, to say that I enjoyed this film would be something of an understatement. In many ways, this is the film I've been waiting some three and a half decades to see. The plot is solid, the action sequences are great, and perhaps most importantly, the acting and characterization are absolutely spot-on - all of the leads have a great chemistry with each other. In particular, there's some wonderful one-on-one interactions (Iron Man fighting Thor, Black Widow interrogating Loki, Tony and Bruce bonding, Cap and Iron Man squabbling, Thor throwing down with Hulk, Tony verbally sparring with Loki) that really highlights their individual strengths and characterizations quite nicely. All of our heroes get their individual moments to shine, and the script does a good job having your 'street-level' heroes (Hawkeye, Black Widow) fit along side your heavy hitters (Thor, Iron Man).
There are some nice touches if you've seen the various films leading up to this, but it stands alone nicely if you haven't. In short, the inner 9-year-old fanboy in me is still bouncing off the walls from seeing it (actually, my outer 44-year-old self is kinda doing the same, only in a more sedate manner).
Oh, and as he was the only lead who wasn't in any of the previous movies, props should be given to Mark Ruffalo, who really nailed playing Banner in the film. As much as I liked Ed Norton's portrayal in the previous Hulk film, Ruffalo just brought a whole 'nother level to the character.
Oh, and if you haven't seen the movie yet, be sure to stay through all the credits. There are two bonus scenes - the first sets up a villain for future films, the other...well, just stay all the way through the credits, okay?