The Freeport Companion: Pathfinder RPG Edition (affiliate link) does a great job of translating Green Ronin's classic setting into the Pathfinder RPG system. I also have the Savage Worlds version of the book, and somewhat prefer the Pathfinder implementation.
The first chapter of the PDF discusses race in Freeport. In addition to the standard races, there is a gnome variant and the azhar, a race descended from the efreet. The azhar may best be described as proud and loud, but they are also loyal, if not terribly far-sighted. The azhar are much like the fire genasi from my D&D 3.0 Forgotten Realms Campaign Setting, in fact. I like the PFRPG azhar somewhat better than the Savage Worlds azhar, and they seem to fit well with the tone of the setting.
The second chapter of the Pathfinder Freeport Companion is about classes in Freeport. The writers went to great lengths to offer some customized classes especially fitted to adventures in Freeport. This is one section that I think is better done in the Savage Worlds edition, since the book offers archetypes, rather than statted-out classes. I don't have any real objection to the new classes (in fact, I love new classes), but for the most part they feel like slightly reskinned base classes.
Despite the fact that most of the new classes feel somewhat superfluous, each does have at least one unique class ability that makes it worth considering. The assassin, for example, has been done many times for many different campaign settings. The Freeport Companion's assassin is done better than most, and I especially enjoy the system for handling assassinations away from the table. The would-be assassin can make a roll to determine the success or failure of his assassination attempt when such an action would distract from the overall narrative or is a regular part of every session. Some outcomes result in success and an improved reputation, while others involve failure and even death.
My final word on classes is that if I were going to use new classes, the first place I'd look would be the Genius Guides. The ever-popular shadow assassin in particular would be well suited to life in Freeport.
Chapter three covers additional rules, like skills and feats, in Freeport. The skills require little adaption, but the few changes made are well-placed. There are also some new languages, which always add fun and depth to a game (says the psycholinguist). The feats are, for the most part, excellent. With so many new options it would be hard to get them perfect every time, but the writers of this particular game get pretty close. It's all about flavor, my friends.
Adventuring is a nasty business, and it often involves brushes with Things Man Was Not Mean To Know. As such, I'm glad that the writers included an insanity mechanic in the Freeport Companion. There are even tables of symptoms that you can choose from or roll on to create fun and nutty PCs and NPCs. You could also use Scott Gable's insanity mechanic from issue 11 of Kobold Quarterly, but I think the Freeport Companion will work just fine.
The equipment section of the book is another fine addition to the Freeport Companion. The firearms are done well, and I especially love the bit on drug addictions. Once again, adventuring is a nasty business and all kinds of peril to life and limb can result. The drugs and poisons offered here will give depth to pretty much any character, though I caution against relying on narrative crutches. When used well, these items will cause - ooh, butterflies!
The spells and magic items of Freeport are equally important. There are few new spells, but the hoard of magic items on offer just make my mouth water. The Reaverbane will make any corsair or privateer tremble with fear, while Ring of the Boar will turn even the most lily-livered tripe into a fine fighting specimen!
The rest of the book is filled with prestige classes, sample NPCs, specific NPCs, a bestiary, and an adventure to start you off in Freeport. The prestige classes fight the general context well without being too specific. You could drop them in just about any nautical fantasy setting or region and it would work just fine. The NPCs and creatures of the bestiary are similar. These are resources that can be used in almost any campaign, but when brought together they make Freeport pop right off the page.
The introductory adventure, Fury in Freeport, is a great way to get started with your new Pathfinder campaign. It's a good introduction to the people and places of Freeport, though it's by no means comprehensive. That brings me to my one quibble with this book. In order to get the full benefit, you have to have the core book. While that's not a huge hardship, it seems silly to require two books to get things going. It does have the advantage of allowing the core setting to remain system neutral, though, so I won't complain too much.
On the whole, an excellent production from Green Ronin. If you're thinking about running an Eberron game in the Lhazar Principalities or just want to try Freeport, you can't go wrong with the Pathfinder Freeport Companion (affiliate link). I give it 5 out of 5 stars. I would dock a few tenths of a star for a couple of minor issues noted above, but in general I recommend this book for purchase! Green Ronin also has a nice print/PDF deal, so I recommend taking advantage of that.