I feel late to the party, but..I went and saw The Avengers yesterday and really really enjoyed it! I was actually worried that it was going to be over-hyped for me. That happens to me a lot. Even if I still enjoy something, it's not as much as others did. Anyway.
For an action movie I thought the light-hearted humor in it was great! I kept thinking, "I'm so glad Michael Bay did NOT do this movie." In my opinion the action sequences were perfect, they were definately everywhere, but weren't stupidly everywhere like he tends to do. Dialogue and casting? Spot on.
Mark Ruffalo is my favorite incarnation of the Hulk I've yet seen. He is all sorts of adorable to me.
I was also glad that they did a pretty swell job of explaining what was going on if you haven't seen the other movies. Because I still haven't seen Thor or Captain America, but didn't feel exceptionally lost with anything, which was really nice. I know I missed some character development by not seeing those movies (which I do plan to see), but plotwise I think I faired pretty well.
If you haven't yet seen this yet, which I doubt, I would highly recommend it! Especially if you love action movies. Joss Whedon, you're a champ.