I liked it, but it wasn't great. I only paid the cheap $5.50 afternoon price, so I feel I got my monies worth. The whole Mandarin not being the real villain is something you can see about three miles away, so when Tony grabs the guy, it's not like it's any great revelation. Guy Peirce's character was about as boring as a villain as I've ever seen, so I couldn't really get into it. The battle at the end was well done, but could have been shorter. The biggest problem I had with the movie, was random evil guy number two flying around in the Patriot suit. The suits are supposedly keyed to specific people, so if you want to get all knitpicky about, that shouldn't have been possible. Since that led to a major plot point, it bothered me more than a little.
Of all of the Iron Man movies, this is the weakest, but it's still a decent enough action movie if you can turn your brain off long enough to watch it. There was a fair amount of humor and some good action scenes. My favorite part was actually the after credits scene with Bannor. Which if you've seen the movie, pretty much tells you how much I liked or didn't like it.