The shorter version: this is the Avengers movie we were looking for.
The longer version:
This is also the Spiderman we were looking for, the Black Panther we were looking for, the adapted classic comic villain were looking for, the Bucky and Cap we were looking for, and the sort of movie the Last Captain America movie ought to be.
This is not War and Peace. This is not Casablanca or the Godfather. This is a movie adapted from the comics in such a way that if you know all the comics, there is a spectacular avalanche of fan service, and if you have never read a comic, the script and action are plenty to satisfy.
I give it a solid 4 stars, and I'll come up with a more-substantive review for it at RottenTomatoes later today.