I saw it tonight, here is my spoiler-free review:
The problem with starting a franchise with a movie as strong as the first Iron Man is that the bar is raised high for sequels. Sometimes they reach that bar, like The Empire Strikes Back or The Dark Knight. Most often they fail to recreate the same magic as the first movie, like 90% of all sequels. Unfortunately Iron Man 2 falls in the latter category. That's not to say it's a bad movie, because it isn't, but it certainly pales in comparison to the first movie.
What's wrong with Iron Man 2? Not a ton of things, but enough to leave me feeling non-plussed at the end. The movie starts strong, and got me jazzed pretty quickly. It starts with the soon-to-be villain of the movie and then quickly switches over to Tony Stark, played with the perfect amount of swagger and charm by Robert Downey Jr. The first act of the movie is solid and has just the right amounts of characterization, story, and action. The problem occurs in the middle act, where the movie grinds to a halt. While I loved the first movie, I did feel it could have used slightly more action. I forgave it for the minor flaw because it was an origin story. I assumed with the origin out of the way the second movie could focus on the action and let us see Iron Man really cut loose. Instead we get an overlong, meandering, and mostly pointless second act that feels bloated. While the third act does punctuate the movie with a nice action piece, it's too little too late because I stopped caring about the story an hour before. I won't list any spoilers, but the movie introduces a completely unnecessary and arbitrary plot point that serves no real purpose. If this one MacGuffin had been removed the story would not have changed one bit. Without a doubt, this was what bothered me the most about the movie; it's pointless and boring. While it wasn't completely torturous to sit through on an initial viewing, I can foresee Iron Man 2 not holding up on subsequent viewings because of this long and overextended middle act.
There were other problems, some bigger than others, but one of the bigger ones is the fact that I never got a sense of heroics or altruism from the main character. In the first movie Tony Stark grows as a character and made my heart soar when he accepts his responsibility as a super-hero. None of that is present here. Iron Man goes from point A to point B, not because anyone is ever really in danger, but because the movie needs him to punch up some action beats. I never experience the thrills or goosebumps the first movie gave me. There was never any real big payoff because there never felt like any real threat, which leads into my second gripe. The movie lacks a sense of danger. Oh sure, there are a couple of typical bad guys running around, but they rarely interact with Stark and their motivations are only glimpsed at. Mickey Rourke was sensational as Ivan Danko (sp?), but his character was completely short-changed. He gets some development towards the beginning, but quickly falls by the wayside. He lacked the complexity and layers that his character deserved and needed. Mickey Rourke does well with what he's given, but unfortunately he isn't given much. Also, it's a minor nerd-gripe, but if he could give Scarlet Johannson some pointers on a Russian accent, that'd be great. Sam Rockwell is a fun villain, playing the poor man's Tony Stark, but again, never quite reaches that peak of villainy that a sequel needs. He's set-up to come back in a later movie with an axe to grind, but he never felt like much of a threat so who cares if he comes back? The movie paints him as an incompetent buffoon. Gwyneth Paltrow reprises her role as Pepper Potts and again, knocks it out of the park. Don Cheadle replaces Terence Howard as Rhodes and I thought he did well, at least, did well the material at handl. I never had a problem accepting him in the movie, he blended in well. Scarlet Johannson is serviceable as the Black Widow. I was irritated she lacked the Russian accent of her comic book counterpart, but she worked well and her one little bit of action is actually one of the stronger bits in the movie.
All that said, while the movie has a lot of problems, it also does things well enough that I'll go ahead and give it the recommendation. I don't foresee it joining its predecessor in my DVD library (until I find it on clearance anyway), but I mostly enjoyed myself. The movie has some really funny bits and while the action is extremely minimal, when it's there it's good.
Bottom line, Iron Man 2 is not a must-see. It's a pretty typical sequel. Still, there are worse ways to spend a couple of hours. Give it a shot, your mileage may vary.