I went to see this today, and I must say I really enjoyed it. The first had some great moments for me (and a fantastic first half), only falling flat with the climactic battle. Thankfully the sequel had a more even tone, even if it did not quite recapture the dizzying and perhaps unorthodox (by superhero movie standards) tone that was set in the origin sequence within the first movie.
The casting was great and no-one felt out of place. I liked the interplay between all the leads and the supporting actors, and Don Cheadle was an improvement over Terrence Howard in my opinion. Probably the most enjoyable performance for me was that of Sam Rockwell as Justin Hammer, basically a Tony Stark-wannabe who is at once pathetic and would-be Machiavelli; a great performance throughout, and hopefully Hammer will be back along with promised appearance of the Mandarin in Iron Man 3.
It isn't high art. It doesn't push the boundaries of the genre in the way that movies such as Dark Knight, Watchmen and Kick-Ass have arguably done, but I really enjoyed it nonetheless, and this time the all-action climax which was always going to be part of the movie felt more tense and fitted better with the progression of the various story-lines.
And to anyone who hasn't been to seen it yet; consider staying for the end credit scene. My friend Wayne was a little indifferent to that scene when he reflected on it to me, and my wife was much more so when she did so after we left the cinema (the only two in the audience who did stay to watch it mind you), but I really got a kick out of it being a fan of comics, comic book movies, and especially the idea of an interconnected superhero movie universe that Marvel is building. No spoilers, but it is one of the longest waits through credits I have sat through for a 'bonus' scene.