It's extremely awesome. I would have preferred that they shortened each of the big action setpieces a bit and used the time for more character interaction, but since that's Whedon's specialty, there's actually quite a bit of that too. It certainly includes the tropes and situations that traditionally made superhero teamups enormous fun, and it deftly manages to pause for serious emotional beats as well as being stuffed with huge amounts of action and comedy.
No time to post at length today, but I will get back to this at some later point. The main thing is, the film mostly avoids the storytelling pitfalls of a such a complex undertaking and succeeds as tremendous fun for anybody who loves Marvel's characters.
Note that there are TWO post-credits sequences! Be sure to stay to the very end.