I just saw it tonight (in wonderful 2D) and thought it was great! I'm a big Avengers fan from way back when I was a kid, and are still reading the comics today, plus of course I'm a Joss Weddon fan, so I'm pretty much the core base for this film.
But ya. Surprisingly engaging action scenes (I think many of the prior marvel films failed on this point), good character bits, and overall a good time. My three year old nephew was also enthralled, for what it's worth.
As for prior films, this draws quite a lot from Thor, so that along with Iron Man 1 would be my primary recommendation for pre-viewing. Captain America sets up the macguffin as well as introducing Cap. Black Widow is introduced in IM2, but you really don't need that at all for Avengers - she's much better handled here than she was there. And IM2 was pretty skippable over-all. I failed to see the last Hulk movie, but as it had a different actor playing Banner, I'm guessing it doesn't set up much that pays off in Avengers. But the actor they did have was quite good I thought, as was the build-up to Hulk's appearance.