I really liked the movie. In a lot of ways, I think it was actually better than the first one. I thought the fights were in general better and more interesting. I loved this version of the briefcase suit. Nick Fury was fantastic in the movie, and I don't think any of the supporting characters grabbed an undue amount of attention or were ignored. A little more Black Widow action would have been nice though. The little references like Cap's Shield were great without being overdone.
I do like how they handled Tony's decent into despair and self destructive behavior, but I did feel that what was missing was what exactly he was doing to find a solution. Pretty much they just had him saying 'Well, I tried everything and since I'm brilliant, there must be no cure'. It seems to me that there were plenty of other solutions, including getting the reactor out of his chest. He really didn't need it per se to keep him alive. He just needed some way to keep the shrapnel out of his heart. And once back in civilization he could have found some other way, or another way to power the electromagnet... Also, he was acting in a very traditional manner to suicidal people and yet nobody really tried to do anything to find out what was wrong than ask some casual questions. Sure he's eccentric, but giving away most everything he owns and doing even crazier stuff than normal should have had Pepper, Rhodey and even Happy more worried than they were.
The villains. Ivan was okay, being a mish-mash between Whiplash, the Crimson Dynamo and some unspecified genius. Neither of the other two in the comics were all that bright. One being a criminal thug and the other a communist patriot both of whom were given high tech toys that they didn't develop. But it worked well enough for the movie, giving Tony someone who could challenge him both intellectually and with armor.
Justin Hammer however.. was a putz. And totally unlike his comic book counterpart. Comic Hammer is a dangerous corporate raider who never gets caught and bankrolls a number of costumed villains to mostly get his rival Tony Stark, who he hates. Movie Hammer is a bumbling fool of a CEO who doesn't seem to know what he's doing, is constantly being outmaneuvered by Tony and Ivan and never seems to have back up plans. He just expects that he can throw some money at a problem and it will solve itself. I have to wonder just how he managed to build a large company, especially if his products are so seemingly unreliable.
The ending and set up for the Avengers movie is interesting, but I won't spoil those here. I've spoiled enough I think. I will warn of course, stay till the end after the credits for another Avengers tie-in mini scene.