Do you like the works of Joss Whedon? Did you like the Iron Man movies, Thor, Hulk or Captain America? If So then you will love this movie and should go see it.
The plot is simple bad guy tries to take over the world, superheroes team up to stop him. Cue explosions. And action. and witty one-liners and off beat comments. (ie whedon stylings)
So Avengers. They've been building up to it for what four years? Since Iron Man in 2008 and boy did it pay it off. Everyone reprises their role from previous films, (with one exception) and everyone pretty much nails their roles. Mark Ruffalo the new guy playing Bruce Banner/The Hulk is very different from Norton but integrates well into the cast as a whole he is more played for laughs than in his solo feature, though the angst is still there more as an under current.
The thing about this movie is the style and the spectacle is a very pretty movie, very fun, very exciting and has lots and lots of explosions from the get go. But what it is not is the dark knight. It's not a serious character piece, its not gritty and realistic exploration or chaos or anarchy or whatever but it is along the same lines of what I loved about the first Iron Man movie. It's so very very <i>fun</i>.
So yeah, like I said, if you like the characters already and if you like Joss Whedon's style of writing and direction the movie is highly recommended but I have a horrible feeling the hype and praise its getting is going to result in backlash against it and well when the Dark Knight Rises its going to get blown out of the water.