I saw Avengers on Monday at an advance screening. It was great. If you wanted to see the characters go all out in battle you'll love end final fight that is very long. I loved it and think every character got to shine and have their moment.
Black Widow was cool and is a bad-ass. Clint (Hawkeye) is very deadly. Capt. America lives up to his character. Thor was powerful. Hulk steals the show whenever he's on screen. ....do I need to mention Iron Man, he's always great in every way.
Only things I can be critical about are The Chitauri army was pretty weak compared to the Avengers and I was hoping for Hulk to say some words, since he did in Incredible Hulk.
Stay after the credits. There are 2 bonus scenes, one after the movie and one after the credits.
I will be seeing this again.