Tony Stark/Iron Man - as played by Robert Downey Jr - is fallible, engaging and humorous, and that just about saves this film as far as I'm concerned. But the story didn't really "click" with me and I found the action scenes to be overdone and ludicrous (even for the superhero genre where ludicrous action scenes are the norm). There are some good (and funny) moments but the whole thing just didn't seem to meld together all that well. Iron Man 1 is, in my opinion, still the best of the three.
I think it's worth watching if you have nothing better to do and serves as a reasonably entertaining 90 minutes or so, but it's no great shakes.