Back to the Future was very successful. It was so successful, in fact, that the decision-makers involved with the film felt that it warranted not just one, but two sequels, which were filmed concurrently. They took us some odd places with the characters, but, ultimately, they gave us the happy ending that we were promised in the first film.
The Matrix was very successful. It was so successful, in fact, that the . . . okay, you get the idea. However, the Matrix trilogy took the cool ending we had enjoyed in the first film and basically threw it in the garbage, leaving many of us, who were so amped up by the potential hinted at in the second movie, leaving the theaters shrugging our shoulders.
In both cases, the studios, and all parties involved, made money hand over fist. In the former of the two, fans of the series were not let down. In the latter, they most decidedly were.
So the "simultaneous sequels" experiment works out very well for the studios and somewhere between "fair" and "not at all" for the fans.
Disney, the home of the "totally-unnecessary-straight-to-video-sequel" (Lady and the Tramp 2, Cinderella 2 & 3) made the obvious choice.
As a result, the third Pirates movie is over-long (2:45), entirely too fond of its own cleverness (hey, let's shoot the monkey out of a cannon!), and so full of absurd character developments and bizarre twists that it makes "The Days of Our Lives" storylines (or at least my best friend's unsolicited descriptions of them) look like an after-school special.
In short, Pirates of the Caribbean: At World's End is a disappointment.
It's also surprisingly brutal for Disney, and maybe even for a PG-13 movie. American morality: crushing human skulls, mass hangings (including a kid), and multiple eviscerations are still family fare, as long as there are no naked boobies. (I would say this even if I weren't the president of the American Lobby for Naked Boobies.)
The positives: the ship battles are still pretty cool (though increasingly reliant on CGI effects), and there are lots of great little "moments" throughout. Though, again, the whole is somewhat less than the sum of its parts.