There's not much to review here, The Avengers was just excellent! I don't think I ever said that about a "superhero movie" before. Each Avenger had their own moment and the action was top notch.
Writer/Director Joss Whedon did an excellent job in combining the characters from all the solo Avengers pics into one.
That's it. Excellent film.
There is a little something more in The Avengers Spoiler Room
Ranking The Solo Avengers Movies