Saw AoU last night... I was very pleased. It held my attention throughout, and didn't feel "stale" at all, despite the similarities to the first film.
I like how they sort of met the conversation about "Hawkeye and Black Widow are not super heroes, what are they doing there?" head-on by showing Romanoff's back story and Barton's hidden home life... it was like they were saying, "yeah, they're not like the others, but they're still Avengers because humanity is powerful in its own right." (I also liked when Barton flat-out said it: "People are flying, the world is ending, and I have a bow-and-arrow. None of this makes sense.")
The humor worked well, too, which I think is important for these movies. They aren't on the same plane of humor as was "Guardians of the Galaxy," but the comic relief is needed throughout the movie. I worried that the humor would feel forced for this go-around, and yet they did an excellent job of timing and delivery. Definitely some legitimate laugh-out-loud moments.
(Sidebar: Would your significant others be as cool as Barton's if you were hanging out with/so close to a female as hot as ScarJo every day? Is there a Romanoff/Barton friendship story from the comics, or do we just buy that they're besties with no romantic interest?)