Iron Man 2 is my favorite movie so far this year. Amazing special effects, wonderful cast and it was based on one of my favorite superheroes.
The plot: Some say it was too slow in the middle. I didn't think so. All the little nods to the comic were very appreciated (Cap's shield, Thor's hammer, scenes from The Incredible Hulk, the map full of locations of comic characters). The story itself is a mishmash of different plot-lines. Little bit of Ultimates, little bit of Demon in a Bottle. Plus some other stuff.
The cast: Sam Rockwell was a show-stealer. I'm indifferent to Rourke because, as #21 put it, "He looks like a cobbler put him together." Great actor, very strange appearance ever since The Wrestler. Johansson didn't have enough screen-time to make an impression on me. Downey was still Downey (AKA: awesome)
Honestly, I loved this movie. Better sequel than The Dark Knight and not as slow as Spider-Man 2. Not quite as over-the-top as Superman II though.