Definitely the most well made Phase 2 installment, an improvement over Thor: TDW which in turn was a big improvement over IM3. I don't think I liked it as much as The First Avenger but still lots of fun.
My only really geeky complaint is Arnim Zola, it was cool he's essentially the puppet master of the movie in a way, but it was a shame to just see him as a bunch of dusty old computers. He's such a cool looking villain in the comics, and while it was nice he returned, it seemed like a waste of Toby Jones.
Also, if Black Widow can't wear a bikini anymore, why do we see her as an underwear model in IM2?
