Most of what I wanted to complain about has already been mentioned, but here are some things that bothered me.
The art has some really disappointing moments. Recoil pads on firearms that have sharp projections facing the wielder would be painful to operate, don't you think?
The whole silly section on communal spells could have been replaced with one metamagic feat. I mean, how is the effect any different from say, extend?
I don't know if any of you have ever fired a shotgun, but shot doesn't leave the barrel in a 15' cone. At 100 yards it's only a few feet wide. It would have made more sense to reduce the number or size of damage dice by range increment. Say, devastating at close range(4d6 of 1d12), but rapidly becoming useless farther away. (1d4, then no effect.) Also, why does the rifle have such crappy range, and do less damage?
I thought the gunslinger was kind of lame, but I'm prejudiced because I have a gunpowder class of my own that I like better.
I liked the armor as DR rules, but I'm not sure why armor becomes useless when fighting big monsters.
What I really liked were the siege and vehicle rules, and it is for them that I have awarded two stars. I wish they'd had more vehicles, and that the ships had floor plans, but so much of the book was devoted to spells (Ultimate Combat, really?) that they just didn't have much space, I guess.
Also, the segment defining different levels of technological advancement was sort of instructive, and offered interesting rules on integrating them into a campaign. I've noted some comments that the weapons cost to much. If you don't want them to be so exotic, the rules state that the prices is reduced in eras in which gunpowder is more ubiquitous.
Mostly I'm disappointed because I expected the book to be about combat. Elaborations/clarifications of the existing rules, new feats that someone might actually want to take, maybe some variants of rules that you might want to consider. I knew that there would be some new classes and such, but that isn't what I was looking for. It really isn't fair to judge Paizo for not putting out the book I was hoping for, but I expected it to offer more.