I'm of the school that believes that early movie reviews should be spoiler-free and not reveal anything about the plot. So feel free to read this and not be concerned that you're going to find out anything more about the story other than the fact that there's a cast of characters and it's a continuation of the first movie.
The recent trend with comic book movie series is to always be bigger than the movie that came before it. A lot of times this has to do with the fact that technology gets better each year, and as a result the imaginations of the movie creators are freed to execute more wackadoo visuals. That being said, bigger is not always better and the results can be a mixed bag.
Successful examples of this exercise are movies like: X-Men and its sequel X2; Spiderman and Spiderman 2, and, obviously, Batman Begins and The Dark Knight. Conversely, less than stellar examples of pushing it too far are: X2 to X3: The Last Stand and Spiderman 2 and Spiderman 3. People will say that either Brett Ratner or Sony ruined X3 and Spiderman 3, respectively, but the fact is that ultimately it was the desires to pack more into the movies, whether it was by way of mutants or villains, that hurt both films.
Where does Iron Man 2 fall on this scale? Is it more of a Spiderman 2 or an X3? IMHO, it's probably somewhere between, maybe leaning a smidgen towards Spiderman 2. While the movie is definitely fun to watch and the actors are all pretty great, especially Robert Downey Jr. and Sam Rockwell, there's still a little of that "We're going to try to fit everything we can into this, because it has to be HUUUUUUGE!!!" feeling to the whole thing. There's a number of rushed scenes, and a fair amount of times when the movie asks you to ignore some pretty decently sized plot holes.
My biggest fear going into this movie was the integration of Scarlett Johansson as the Black Widow, and I was really pleasantly surprised by the results! This stems mostly from the fact that her normal go-to role is "free spirited"/slutty girl who poaches a guy just by way of her curvy-ness and independent-ness, and that wasn't something I really wanted for either my superhero movie or Pepper Potts.
So if you weren't really going to see Iron Man 2 because you didn't like the original Iron Man and didn't understand why they made a sequel for it (answer: because the first one made like $500 million), then you probably shouldn't go see this movie. But if you're a normal person who likes things and also enjoys a good superhero romp at the movies, go check this one out!
Also, Christiane Amanpour kills it in her cameo. If the talks of spinning her off in her own movie haven't happened yet, my guess is that they'll engage in those conversations soon.