There are several things that make the Iron Man films the best comic book adaptations, alongside with X-Men.
1) Robert Downey Junior
2) Great, even awesome, FX
3) Humor and cynicism
4) Realism: Tony Stark doesn't have superpowers. He built an armor from scratch, "anyone" could do that, sort of. That's the big difference between Iron Man and, say, Spiderman or Superman, who get their power from magic sources (okay, mutant spider and foreign planet) and are therefore "chosen". Tony Stark has special skills before he gets his power by wearing his armor; he chooses his fate.
>> Assume spoilers from this point on <<
I loved Iron Man 3 for its FX, although they could have gone further with the 3D.
I also loved the the ongoing characters' follow up story. Pepper & Tony are a great couple ; the government and army characters & stories are always realistic enough to be believable ; the cute but not naive kid was a great addition to this opus, the decoy villain was simply hilarious and fit extremely well in the Iron Man universe.
What I didn't like, though, was the main villains: the whole "growing a new limb" thing was, in my opinion, too far-fetched to fit the Iron Man-verse. We went from fighting Middle-East terrorists (1st film - Enemy #1 of the US, It'd say that's realistic enough) and a Russian challenger (2nd film - again realistic) to WTF mutants (3rd film - where do these come from?).
I'm usually all for the films which do not explain everything (hello? We're not dumb!), but in this particular case I think the whole mutant thing was shaky: in the end, we know how they were created and why, but not how to effectively kill them. Shoot them? Some die, some come back. Explode them? Some die, some come back... that's not solid ground for a villain. And though they tried to give a physical explanation on how the mutants were created, I didn't really fall for that.
In the end, that was a great film because what was built in the first two films remained and continued to evolve. But I guess I think of the villains as an accessory.