To be straight forward, I thought the movie was good. Not mind blowing, not terrible, just good. The special effects were great as always, as was the acting (with the possible exception of the somewhat comical scene wherein Tony Stark has a toddler-like meltdown while having nightmare flashbacks of the final scene of The Avengers). I have to say that I would have liked to see more of Tony Stark in the Iron Man suit, but without giving too much away there are various reasons why this wouldn't have worked with the story line. Guy Pearce and Gwyneth Paltrow both gave commendable performances as supporting cast, and we see a lot of familiar faces throughout the film (with a comical scene in the after credit scene at the end of the movie with Mark Ruffalo).
I think the biggest problem I had with the movie was the more "fantastic" element of this Iron Man movie. Now, let me explain what I mean by that. I am perfectly aware that all of the Marvel movies are outlandish - that is a part of the reason we love them so much. As ridiculous as the possibility of gaining spider superpowers after being bitten by a radio active arachnid is, everybody loves the idea of swinging from New York skyscrapers and climbing up wall while fighting super-villains. That being said, I think the reason that Iron Man is and always has been my favorite Marvel hero is that I always felt that his story was one that a more grounded in reality than the others....well, as grounded as a man in an invincible suit of armor can be. The idea that Tony Stark was a super hero because of his genius and technological prowess rather than radio active mutation or coming from another realm of the universe made him that much more likable to me. Without giving too much away, I felt that part of that reality was lost in the third movie as the villains largely consisted or chemically mutated humans with regenerative super powers.
With the villains of the past two movies being normal people that combated Stark using his own methods - namely money and technology, the movies seemed to be more enjoyable to me because, as far fetched as the movie was, it was more realistic than all of the other heroes in the Marvel universe. Perhaps I am just being too particular (which I tend to be when I am watching a movie, especially for the first time), but I felt that as the seemingly final installment in this version of the Iron Man movies, it didn't quite match up to its two predecessors. Don't get me wrong, I liked the movie. In fact I will probably go see it again in theaters, but I felt that for Tony Stark's last hurrah, less would have been more as far as the mutant human firebombs were concerned.