I just got home from screening The Avengers 3D, and I can say with no hesitation that it is one of the best comic book movies made thus far.
The Avengers unfolds exactly like a comic book: The first act shows the villain moving his pieces around and introduces each individual hero. The second act has the villain carrying out his plan as the heroes come together and (in true comic book fashion) fight each other. In the last act, the heroes finally band together to combat the threat. It is a simple formula, but a very effective one.
If you're a fan wondering how characters such as Black Widow and Hawkeye fare alongside the icons of the Marvel Universe, don't worry. They both have fairly detailed backstories and get major screen time. In fact, the same can actually be said for each and every character. The result is a fairly complex set of motivations and interactions between all the players.
One quick note about the 3D. It really enhanced several of the scenes (specifically the battles), but for the most part, it wasn't that noticeable.
Go see The Avengers as soon as it opens because it is completely Badass.
