Caught the midnight showing of the Avengers last night. I won't get spoily on you, because I hate them myself, but I will leave you a few tidbits:
1) The Hulk totally steals every scene he's in. Mark Ruffalo totally nails it as Banner.
2) There are TWO easter eggs in the credits, not just one. Stay until the lights come on.
3) The movie rocked. People stood up and cheered a few times. (Ok, it was mostly me, but seriously... GREAT movie.)
4) Yes, there are some comic relief scenes, but they are (IMO) well timed and not overdone.
The one I saw was in 3D, and it was "Avatar style" (meaning they were going for depth of field, rather than cheesy in your face stuff.) Was it absolutely worth the extra money? Probably not, but the 3D trailers before it were pretty sweet. (Trailers: Prometheus, Spider-Man, Batman, other crap. YMMV)
Joss Whedon ala Buffy, Angel, Firefly, Uncanny X-Men (comic book), etc has produced an EPIC movie (and not in the geek slang way... but in the true sense of scale.) This is a story that would have potentially played out over a year or more in the comics, across multiple titles.
Go see it. Go with your friends. If you don't have any friends, sit near a group. The fan and audience reactions enhance the overall experience.