I knew that I had to own a copy of this book the moment I heard the Gunslinger Base Class was included in it. And I'm actually very happy with it. I think Grit is a great mechanic, and it's even better when your GM can use it as a reward when the Gunslinger tries to pull off some crazy maneuver and succeeds.
But as a GM who is struggling to find a way to fit firearms into a world without completely breaking it for our plate-mail friends, I was disappointed. I understand that the aims of the firearms rules were meant to be realistic (and even includes a disclaimer), but I still weep for our group's fighter when I read these. Nonetheless, the rules for firearms can certainly be tweaked to help the armor-bearers of the group. I simply gave them half their armor bonus (if they had med. or heavy armor) as DR, rounded up. Realistic? Probably not. Keeping the fighter from throwing the book at me and encouraging him to keep his head down? Absolutely.
That aside, I had high hopes for the Armor as DR system for Pathfinder, and I have to say I was disappointed also. I'd been hoping for 3.5 to do the idea justice, but they never even came close. Pathfinder at least makes an honest attempt, but still falls short.
Monk Feats? Awesome.
Steampunk Gunslinger Wizards? Awesome. Can't wait to test 'em out.
New Archetypes? Always fun. Really like the Musketeer Cavalier Archetype. It really encourages me to run a Commonplace Guns Pathfinder setting (once I settle on the firearm rules)
Piecemeal armor & gladiator weapons? Great if you're planning on running any sort of Arena games. If not, then you have some detailed armor for bandits and such.
I feel the Eastern themes of this book could have been played up more, Let alone the lack of Samurai and Ninja Base classes. At least the weapons were very detailed and interesting.
All in all, I see this book as one long list of options for use in your own game, but it's not a book where you will use every shiny new rule bauble immediately. I think the book is more subtle than that, and encourages the players to come up with their own use for the material given in this book (after all, the spellslinger archetype is so out there that I feel like creating a world that could explain it as a variation or an evolution of the common wizard.)
I would say this book might not be completely essential to everyone at it's 40 dollar price tag, but if you want to add some bits (read: boulders) of flavor to your game, you can't go wrong with this book.