I've seen the film twice now, and I'm going to do my best to avoid pointless hyperbole in reviewing this.
Short version: it's tremendous entertainment, confident and complete in a way that none of the Marvel movies so far have been, and I say that as someone who likes the Marvel movies in general. The company makes an incremental leap forward with this movie, and they've set the bar fairly high for themselves in the future. I am pleased and impressed and feel like this more than pays off any emotional investment I made in the movies as they were being released.
When discussing "The Avengers" as a film, though, there are several ways to approach it. You can look at it as a further evolution of what Joss Whedon does as a writer and director. You can review it based on its place in the Marvel canon overall. You can analyze how it fits into the overall genre of superhero films. I think the only way to place it in the correct context is to approach it from all of those directions, because the film seems to occupy a unique place in pop culture, and considering how big a commercial product it is, there's something sort of revolutionary about its very existence.