The Blu-Rays are definite upgrades over the DVD's, especially the first which didn't have a great DVD transfer. The third movie seems to benefit the least but it certainly does benefit. Just take a look at some comparison shots.
Curse of the Black Pearl
Dead Man's Chest
At World's End
One note about the extras: The third movie originally came with a bonus disc at Best Buy. These extras are not included on the Blu Rays:
At World's End Best Buy Bonus Disc