At its high points - the first act and the climax - Iron Man 2 is actually better than the first film. Everything up to and including the action scene in Monaco is just great fun to watch: the action, the character interactions, and of course Robert Downey Jr's wonderful portrayal of Tony Stark. And the action scene at the end is pretty epic.
The problem is, the film just stops being so much fun in-between. In a large proportion of this time, it's either going too slowly with little happening that's exciting or even particularly interesting, or it's providing some silly moments like Iron Man lounging on a giant display donut. To be fair, there's no problem with the many subplots this movie has: they all blend together quite smoothly.
The acting in this film deserves credit: almost everybody does a great job. Robert Downey Jr and Gwyneth Paltrow are just as entertaining as they were in the first film, Jon Favreau gets more to do as Stark's chauffeur, Don Cheadle is actually a little better than Terrence Howard as Rhodie (again, maybe because he has a bigger role), Mickey Rourke portrays a decent but overall ordinary villain, and once Scarlett Johansson is allowed to do something substantial with her own action sequence, she's well worth watching. The only weak link is Sam Rockwell as Justin Hammer, who gets quite annoying after a while.
Overall, Iron Man 2's slow middle section prevents it from being better than the first film as a whole, which is a shame considering how brilliant the beginning and end segments are.
