Trying not to replicate the other reviews, this campaign setting is exactly that - a sketch of many different cities and city-states within a loosely defined country. The locations are creatively detailed, with the intent to provide a HUGE variety of rulers to parlay with as players build their own domain during the Kingmaker campaign. For that it works brilliantly! There are regions to trade with, city-states to provide problems for the new PC rulers, leaders to ally with, and locations to be explored. As I read through this sourcebook cover to cover (despite often finding it difficult to stay engaged with other PDFs), I couldn't help but think about the possibilities for many of the areas.

Admittedly, as many of the other reviewers stated, this is light on crunch. If you are interested in stats, stay away from this product as there are only two gods with basic worshipper info, 5 new poisons, 1 spell, 1 alternate bard class ability, 1 region affinity, and 1 magic salve in the 32 pages. However, the nature of the sandbox campaign compliments Paizo's approach with this product: giving a huge range of idea-generating options for the GM to flesh out. The only thing I found wanting is the region to the east, immediately abutting the Stolen Lands, was not detailed as much as the other areas surrounding the campaign. However, that being said, if you are going to play Kingmaker I think the number and variety of kernels of adventure-generating encounters far outweighs that gripe.

This is a somewhat strange campaign setting book, in the sense it seems not only is the various regions described in the book very different (from very regular fantasy tropes and/or very real world inspired) to the very fantastic... But how they are described also varies so much, because the authors do their pieces in very different ways.

For example, one region might have a city statblock, a lot of info on people there that can be usefull and inspiring, written in an entertaining way... While at least one other is pretty much "there are these guys guarding something but I won't tell you much about either" over three pages.

I found some of the articles in the book excellent, some average and some bad. As there is something to every taste here (except crunch, this is pretty much pure fluff), I guess that would be true for most people too.

Indispensable with the Kingmaker Adventure Path, The River Kingdoms reads like a fantasy novel sampler kit; look who wrote it after all. My only complaint? Why did they limit it to 64 pages? This could easily have gone much bigger...adventure path sized. Check my full review Guide to the River Kingdoms

The River Kingdoms is one of the more interesting settings for Golarion. Paizo does a nice job of keeping things detailed just enough to give creative GMs some leeway to add in some details of their own.
Being a big fan of China Mieville, I greatly appreciated his contributions in the description of the unique city state of Outsea.
There are some great sites of interest within one of the more standout areas of the campaign milieu that the PCs can explore. For example, a town that is a town but not a town... Can't say more as I don't wish to spoil it for any upcoming readers.
If you're thinking about using Paizo's next AP- Kingmaker, I strongly recommend purchasing this sourcebook.
I too wish there were more NPCs in the sourcebook but the material was an intriguing read.

This book deals with the River Kingdoms, an area crammed with independent states, countries, fiefdoms, cities, towns or even villages. Setting up such a patchwork allowed Paizo to bring a whole host of authors to write about 22 areas, each of them different and unique.

And the writing is, wow, excellent. From RPG veterans (John Wick, Chris Pramas, Richard Pett, the Paizo Crew), rising stars (RPG Superstar winners) to veterans of fiction (Elaine, China) the line-up is amazing and the quality rocks the house. The areas are unique, vivid and sometimes plain outlandish.

However, the book has a few shortcomings. Presentation of the chapters is not standardized - some authors provided city statblocks, flags/coats of arms while others kept to pure fluff. The map is somewhat unclear and lacks detail - a poster map would be a far better move.

Also, while the books is written with background material in mind, the amount of crunch is very, very small. There are next to no feats, traits, spells or equipment.

Overall - great writing and excellent ideas with some slight presentation issues and a dire need for more crunchy bits. Still, it's a fantastic book and I recommend it wholeheartedly.