I must say my hopes and expectations were so low after the second movie that maybe this movie isn't really as good as I think. Having said that, I saw it last night and was most impressed. With multiple double-crosses and alterior motives on the part of, well, everyone the entire movie is one big 170 minute Mexican standoff.
I don't think it was just lowered expectations - I truly think this was a great movie. If you like pirates. And all of these great actors. And good storytelling.
Normally, I'm not an action & adventure guy but I did enjoy the action sequences more than usual. Sadly, the comedy falls flat in most instances. What's interesting about this series is it started off so tongue in cheek. The harder it tried to be serious adventure and all too self-aware comedy it lost something.
What's rather weird is I think you can watch the third movie in either Lord of the Rings or The Matrix without watching the first two films of either yet this one you actually need to know what's going on if you want to see it.