It's definitely an Act II (i.e., a "middle chapter") and unable to stand on its own -- it requires the first one to establish the context, and the next one to finish the story.
But that's not a bad thing ... it's just as true of, say, The Empire Strikes Back.
I particularly like that it just picks up where the first one left off and addresses the ramifications of the first movie's ending; you can't just let a condemned man walk and not get heat for it, even in a Disney movie. ;) This transforms PotC from a one-hit wonder into a very cool piratey epic, which the world has needed badly for years!
I will also say that the big surprise at the end was a very good twist. Usually I can see movie surprises coming a mile off, but this one got me. It works perfectly well, it's not so far out of left field that it makes you want to say "I Call No Way!" -- but it was something that I just would not have thought of.
So yeah, it's good, go see it! Just wear plenty of eye protection, 'cause there's a ton of plot threads flying around!
PS: The trailers before the movie were decidedly disappointing. The only one that didn't leave me hostile to the movie in question was Transformers, and all I have to say about that one is that they seem to be taking it awfully seriously for a movie about robots from space that disguise themselves as cars.