This is much more of a sequel to THOR and CAPTAIN AMERICA, though takes into consideration the two IRON MAN movies and the most recent INCREDIBLE HULK film.
The premise is that S.H.I.E.L.D is studying a powerful energy source from Asgaard and Loki shows up and steals it. Banished from that mythical kingdom a year before, he's found alien allies and together with them wants to conquer Earth. Hawkeye is brainwashed into becoming Loki's henchmen and when Black Widow hears this she's ready to go to his rescue.
Nick Fury approaches the Superheroes and tries to get them into a team, though there's lots of fighting, with Iron Man fighting Thor, Hulk fighting Thor and Iron Man continually quarreling with Captain America. All of this works. What doesn't work is a long scene in which the Hulk chases after Black Widow-- it's inconsistent with how the Green Goliath behaves before and after this scene and doesn't do anything for the overall story.
The 3-D effects actually work well, particularly with all the flying--of the characters, spaceships and S.H.I.E.L.D.'s impressive heli-carrier.
Of course, this version of The Avengers is more in line with Marvel's THE ULTIMATES series than it is the original story (I miss Ant Man & The Wasp!), with a bit of humor thrown in. It's worth seeing just for the Hulk/Loki fight. Overall, this Joss Whedon directed movie was well worth the wait. Highly recommended.