The short version: it's good, but over-long.
The first half-hour, as siggav pointed out, could be an entirely separate film; the story doesn't start until that whole sequence is out of the way. It only serves to bring Wil and Sparrow back together. There's also a long sequence on Davey Jones' ship were not much dramatic happens for quite a while - to the extent that I was thinking, hmm, not been any action for ages.
On the other hand, the mill-wheel sequence is breathlessly superb, intercut with other goodness.
The story rumbles along quite happily, and it all looks good. I'm not so keen on the acting, though. Depp is a joy, as ever, while Bloom just looks earnest and Knightley looks arch. Heck, I found Bloom more convincing than Knightley, apart from right at the end. Nighey was slightly disappointing, too, in that he over-egged his facial expressions. Probably for fear that they wouldn't show up under the CGI, but having seen the subtleness that Katsulas used in his portrayal of G'Kar, I wanted more.
Having said that, Davey Jones is a good character, and I have thoughts about where he's going to go in the third of the trilogy. But the revelation was Davenport as Norrington - a far more interesting character, and beautifully done. Nicely transformed from the previous film.
Good stuff. Want to see it again, to re-consider everything. Luckily, I plan to do just that. :-)