Should you go watch it?
Yes. If you liked the previous ones but you won't be blown away.
Spoiler free review.
It's good but I didn't love it and I can't pin-point good reasons for why not.It's very iron-manish, very tony-starkish, very robert-downey-jrish and so on. It has the jokes, the special effects, the continuity with the other movies and it's entertaining.
Even the one joke they borrowed from Avengers and overused made me laugh every time. It was basically reading a comic book saga, just not one of the great ones.
I'll say what bothers me in the spoilerish section but, if you haven't seen it, go watch it. Just don't expect it to be faithful to the extremis saga.
So: the order of Iron-Man movies "quality" is: 1, 3, 2.
Note: 3D is cool at all action scenes.
But again, that's just an assumption and if the idea is to show Tony Stark more than Iron Man, I'm cool with that.
Another thing that bothered me a bit is that the trailer was tyring to misslead you all the time. The iron patriot is not the comic's iron patriot (well, we all knew that one), but extremis is not really extremis, AIM is not really AIM and the Mandarin... well, you know if you watched it.
The problem is that I don't read Iron-Man and I felt misslead, I imagine real Iron-Man fans having a big problem with this.
Other than that, can't give you a good reason why the movie is bad and some things just look great.