Joss Whedon’s treatment of The Avengers is pretty faithful to the source material and a lot of fun. I wouldn’t rate it quite as highly as some reviewers — frankly, some of the early action sequences bored me, and I thought the film was a bit too long — but the good stuff, especially the sense of humor, more than makes up for the bad.
It’s of course an ensemble film, and it manages to be surprisingly balanced, with every character with the possible exception of Hawkeye getting his or her due. Iron Man, Captain America, and the Black Widow wind up a little ahead of the rest, but not by a lot.
Be aware that there are two extra short scenes after then ending, one halfway through the credits and another at the very end. It’s not the end of the world if you miss either, but I enjoyed, brief, understated second one in particular.
(Egad, I'm actually posting a review of something actually still in theaters...)
(Very often, in fact most of the time, the trailer is more entertaining than the movie, but for some reason none of the ads I've seen for The Avengers does the film justice. This was the best of a bad lot, at least of those I found in a few minutes of hunting. Don't avoid the movie based on a trailer than makes it look more brainless than it really is.)