Iron Man 2 is one of those movies you're either planning to see or you're not interested in it at all. Certainly if you liked the first movie (and a lot of you did) you'll go to this one. And I don't think you'll be disappointed. The first movie was the origins story. This one is the crisis of confidence story. Maybe you wouldn't think that a guy like Tony Stark would suffer from a lack of confidence but he's got real problems here, problems that Iron Man can't solve. And I'll give Robert Downey, Jr. credit, he does a pretty good job as the trobuled Tony Stark.Samuel L. Jackson is back as Nick Fury, with a larger, although tangential, part of the story. Scarlett Johansson is there to look good and kick ass. You've seen her do some of that in the trailers. Gwyneth Paltrow reprises her thankless roll as Pepper Potts.
In the reviews I read a lot is made of an actor with the stature of Don Cheadle being in the movie. I haven't see too many of his movies so I can't speak much about his chops as an actor. In this he mainly gets to look pensive. But most of the time he's in the big War Machine suit with the heads-up display reflected on his face. Not much room for acting in there.
Mickey Rourke is pretty good as the mad Russian scientist and brute who designs his own suit and almost brings Iron Man to his knees. Sam Rockwell plays Justin Hammer, the owner of Hammer Enterprises, Tony Stark's biggest competitor. I was not real impressed with this stereotypical character as portrayed by Rockwell.
The movie is quite preposterous, of course. You'll be suspending belief all over the place. Why doesn't Tony Stark end up as a bloody paste after some of the punishment he takes in the Iron Man suit? How is he able to build a miniature cyclotron in his living room, overnight, from parts he found in his garage? How does his chief of security get repeatedly punched in the face and not have a mark on him? Why is Gwyneth Paltrow completely wasted in this movie? Stop asking yourself those questions and enjoy the movie.