I picked up the PDF, overall not a lot of changes in the classes. Assassins got a HD increase, Corsairs got a HD increase, Monster hunter had the biggest change adding a few benefits to help him fight his enemies, survivor got a HD increase, no real changes for the noble or cultist. There were a few issues with the feats conversions and they did not change much from the D20 version. They also fixed the skill points.
I did not spend a lot of time looking over the stat blocks on the characters in the back but they did make some changes.
Overall I think one more edit would have fixed some minor errors that slipped through but it was okay.
If you have the d20 version you probably do not need an upgrade, but if you do not have the old version I would recommend picking up the pathfinder version.
There are gunpowder and sanity rules but NO SHIP RULES. If you want those I would recommend Skull and Bones also from Green Ronin. Otherwise you will have to use one of the other 3.5 sources. I am not aware of any ship based rules of pathfinder yet.