(In England, anyway.)
The following review, while spoiler-free, has explicit spoilers for Thor as well as potential implied ones for Iron Man 2 and Captain America. That said, if you’re reading this review and haven’t seen those yet, go watch them and then come back here.
The thing about The Avengers—or Avengers Assemble, which only about ten people actually call it—is that it was pretty much guaranteed to be at least decently good. Obviously, until it came out, there was no gauge of exactly how good it was, but like macaroni and cheese or chocolate, for it to actually be bad, something would need to have gone pretty spectacularly wrong.
(And it’s not, strictly speaking, a sequel to anything, so I’m going to avoiding pointing out that, often, things do go spectacularly wrong for sequels.)