Not as good as the first movie. The Marvel movies tend to be overstuffed, and this was no exception. I kinda wish they’d cut back on some of the action sequences, particularly (mild spoilers) the Hulk’s brief rampage in a “African City” (end spoilers). At least they twisted it for the final fight (because, as a Marvel movie, there must be a climatic fight) where many of the Avengers were trying to protect people instead of, say, destroying a city (again).
On the plus side, it actually picked up on some of the plot threads left dangling by the first movie, like… Do people even like the Avengers? What happened to all the Chitauri tech left behind in New York? Is SHIELD good or bad? The movie makes room for some quiet character bits, which were all very groovy. I liked all the Black Widow, Bruce Banner, and Hawkeye stuff, and the recognition that being an Avenger (well, a superhero, really) takes a toll on people.
It’s biggest failing — entirely obvious, of course — is that it’s the transition between rounds two and three of the Marvel cinematic universe. It’s not so much its own movie as a set up for the next Iron Man, Captain America, Thor, Avengers, (and hopefully) Black Widow flicks. The first Avengers movie was great because despite taking characters from the previous movies it was its own thing; this movie never had that freedom.
(But it’s still good. Go see it.)