I am largely unfamiliar with the Marvel comics about Iron Man, so there were a few moments where I felt like they were making a joke I wasn't expected to get, but only a few. By and large I felt just fine appreciating this movie within the franchise only, without worrying about the comics.
And I appreciated it greatly. Often, the second movie in a superhero series is the weakest, in part because the "character origin" story is usually the most compelling part. But this movie pretty much entirely lives up to the previous.
It picks up from the situation that the last one ended on, and rather than recoiling from its implications, it revels in them and milks them for all they're worth. They give us a lot of great opportunities to see the same conflicts and struggles -- Tony being far too much Tony, the people around him trying to deal with his excesses, the struggles for discovering and perfecting things, and the plot twists. But they gave a new spin to them.
It's true that the story is a little more scattered in places, notably in Tony's personal arc, which seems to be split between several storylines that are only tangentially related. On the other hand, the first movie's story is scattered in other places. I particularly felt that the final battle in the first movie felt a little disjointed, and weakened the movie's arc; no such problem here, the final battles were both coherent and exciting. Overall, the part of the movie that dwells on the opposition felt stronger this time than last time.
The casting of Sam Rockwell as Justin Hammer was the shining light that elevated the entire movie, and Sam stole every scene he was in, except for a few against Downey, and even then only because the script kept him from doing so. I sure hope Hammer's story continues in future movies.
By contrast, Don Cheadle, taking over for Terrence Howard, seemed flat, and hardly worth the change of actor. (Though they winked at the audience about the recasting during Don's first appearance, which was amusing.) I don't blame the actor; while his character had more to do in this movie, the actor had less to do. But Don did play down what he did get to work with, too.
The scenes featuring SHIELD, building no doubt towards some future story arc (whether in an Iron Man movie or an Avengers movie), felt a bit forced. They didn't really belong in this movie, they served little purpose but setting something future up, without fitting in. The only exception is Scarlett Johansson's character, who was dreadfully underutilized -- more's the pity, because some of her action sequences were probably the best in the whole movie. (I say "probably" because the cinematography, as usual, prevented us from getting a really good look.)
Some people dislike Gwyneth Paltrow, either generally, or in this specific role, but I don't see why. My only dislike is that she seems obligated to be put in peril at least once a movie so she can be rescued, even if she also makes clear it's not because of some innate frailty, but just the dumb luck and the circumstances of her life. I really like how she plays Pepper Potts, even if the role is pretty small.
The comedy in this movie didn't quite come off as solid as in the previous one, in some ways reflecting Tony himself going into a darker place in his life. But there's still enough to keep the movie flowing, and still more than one usually sees in superhero movies, all of it still feeling quite organic.
If you liked the first movie, this one might not quite live up to it entirely, but it's fairly close, and definitely worth seeing in the theater.