There are oft repeated complaints about the movie too much stuff/plot/characters, a lot of talk and repetition of things already done in the previous movies. Actually, these are the movies strengths.
It's a movie by fans, for fans. Well, I'm not sure if these are the proper terms it's a movie by someone who loves the setting and characters, for people who love the setting and characters. It rewards you for paying attention to minute details, including those from the last movies, as well as character motivation and mythology. It does not reward ignoring the "talky" parts while waiting for the next action sequence and then getting confused about the "overtly intricate" plot.
Yes, nearly every plot hook, character, and plenty of gags from the last movies are brought up again. The plot threads are more or less neatly tied together, each character gets the fate s/he deserves and the gags ah, those, are where the movie really inherits a great boon from the last two films. Each gag is used once or twice, to the fullest potential, and then dismissed before it overstays its welcome, or taken into an entirely new context. Only with that kind of legacy can a movie send an audience into fits of laughter merely by having a character beckon a dog to him and say "Sea Turtles".
Every element has been fine tuned to near perfection. There's only one full blown swashbuckling/naval combat scene at the climax, where it belongs. And having waited an entire movie for it, I find it hard to imagine an audience member that would not be overwhelmed and swept along. All the double, triple and quadruple crosses are in place, and they are fascinating. Not only are you always unsure of just which side each character is playing, but the negotiation and parlay scenes are filmed in the same manner as the action scenes and it WORKS.
Comparisons to Return of the King are nearly inevitable, regarding various aspects of the film, but the most pretient, in my opinion is that this is the first movie in the series which actually creates the feeling that the heroes and the action are in another world, one of swashbuckling myth and wonder - the sort of feeling you got from any given LoTR movie.
Another RotK comparison (as well as a favorable comparison to the first two Pirate movies) - the movie is long. Some scenes could have been shorter. But there are absolutely no scenes that feel as though they are going on too long. There are no "fight on the wheel" or "drowning Will" scenes that go "meanwhile, are heroes are doing the exact same thing they've been doing for the last 5 minutes".
Shortcomings? The huge chunk of unwieldy exposition shoved into the beginning of the movie, a bit of overacting from Rush, and Chow Yun-Fat's characters should have been given more to do, or cut out altogether. Other than that, this film is excellent, and a fitting end to the trilogy, right down to the humorous/bittersweet/ the road goes ever on and on, ending.