OK, I have to admit that at the end of Iron Man 2 and after Captain America and Thor (both of which were only barely watchable), I was wincing at the thought of the Avengers. I thought it would be another awful adaptation and mincemeating of another of Marvel's properties. Except for one thing. They hired the Whedon.
And the writing and dialogue and the interweaving of the origin stories, character tensions and the history already revealed in the other movies is what makes this film. What worked in Buffy and Angel and Firefly, an interleaved and progressive story arc where history has consequences, makes this a fantastic movie experience. Oh sure, the special effects are incredible and it's got big stars, but it's the story and the dialogue and interactions between the characters that make the film, and that, well… that's Whedon.
Basically, just go see it. And see it in the theatre. It's worth it. It's the first time in so long I've actually thought a movie was worth every penny I paid for it.