I'm going to be saying things that others have said--but it really is a shame this movie isn't better! The first Pirates was so cool, such a great adventure, but the sequels should be making Disney blush. I'm sure they don't care, however, so long as the box office numbers are high.
The sequels served to make Captain Jack more of a buffoon and less noticeably savvy. The plots are ridiculously convoluted, hard to follow and illogical. I admit to finding Pirates II a lark even as it got too silly to be respected. Pirates III is worse. Too long, too inexplicably bizarre in places (I'm thinking of Jack in the beyond, fantasizing his multiple selves). Keith Richards' appearance was a let-down when it came; more could have been done with his character. Keira Knightly gets less cool and likeable with every encounter; she's really only window dressing. Her supposedly rousing speech before battle is trite & substanceless. Poor Orlando Bloom is overshadowed in all this. The denouement, in which the East India Company baddie has some kind of mental breakdown and doesn't fight back, rings false. I suspect it was reached for because the rest of the film had already taken up too much time and they had to start thinking "wrap."
Having said all that, the film is a visual feast and however much of a buffoon, Depp's Captain Jack is worth sitting through this nonsensical mess for (though maybe only once. I went a second time and it was just too much, seemed even more tired than the first viewing). I love the excellent Geoffrey Rush as well, was glad he'd been brought back (though it was on a very thin plotline). Bill Nighy's Davy Jones is also a treat.
I dread the trotting out of a Pirates IV...though if it comes to pass, I'll probably see it, so long as Johnny Depp plays Jack.