Saw Pirates 3 today. It was much better than Spidey 3. Key to this is the fact that Priates 3 actually had an interesting plot, and developed some characters who had been neglected somewhat in Pirates 1 and 2. Both K^2 and Orlando Bloom had a lot more to work with in this film than in either of the first two films, and in the case of Bloom, he had a lot more to work with. K^2 got to walk away a bit from her Good Girl image that was central to her character in Pirates 1 (but not so far away as to offend the public and kill the box office).
There were also a couple surprising plot twists, which is very rare in the tentpole/blockbuster genre. Not a film that I'll see again on the big screen, but on the whole a worthy addition to the trilogy. Indeed, I would say that the Pirates trilogy has maintained a decent quality level, a feat missed by most trilogies (Godfather, Star Wars (each trilogy considered separately), X-Men, Spidey, Lord of the Rings). Where Godfather II has always been the standard for the best sequel ever, I think Pirates, though not quite at the same level in any of the films, should show the way to how film quality can be sustained.
(About LotR: I really found the third movie to be cutesy and tedious. There was a steady decline in quality from the first to the third film, and that's too bad. If Peter Jackson had held his excesses in check a bit more, the entire trilogy could have been a masterpiece. As it was, I had to cringe at the ridiculous fight scenes in Return of the King, and then endure about five different moments where I said "the film could end now. Yes, part of the latter problem was because of Tolkien's writing, which also peaked in the first book. But a good director would have cut out the trip back to the Shire.)