This is overall a fairly useful resource for GMs, though some parts of it strike me as a little strange and/or off. The stat blocks alone are worth the $4 and are a HUGE help to the GM that wants to use wererats and wants more than just the one bestiary example, but hates sorting through all the rules on that template. This is kind of a long book with a lot of parts, not all of which I used, so I'll share some thoughts on what I did use. I should start by saying I did not use this book for its "tribes" purpose in a city/sewer, I got it for the stat blocks hoping that I could use them for an idea I've had for a while now for a pirate ship with a wererat crew. I didn't use the first nine pages at all, though some of it (sewer features/hazards and semi random encounters) would be useful for a city setting.
Page 10-11 has new class features, used in the book's NPCs, but not very well suited for PC use. The Filth is a cool concept (a kind of "ideal" that a cleric can devote herself to to get a domain, and that a witch can use as a patron, but not an actual deity) and makes for some nasty NPC devotees. The sewer druid is a variant blight druid whose abilities (mainly sewer stride) work extremely well on a ship, as well as (I would imagine) in a sewer. The rogue talents and one of the rage powers are awesome, though the "roach run rage" rage power is only good for wererat on wererat action. Feats were all sewer specific, so I didn't try them out (i changed feats on the NPCs that used these), they seem ok for sewer based NPCs, but very niche specific.
A few of the spells are basic, almost copy and paste from Paizo. Detect lycanthropes is just detect undead for a different creature type, detect vermin even says it functions as detect animals and plants except that it detects vermin, and hide from vermin is a modified hide from animals. Forced transformation is a potentially useful debuff against a lycanthrope in hybrid form, though I think it might do better as a close range spell rather than touch, since (IMHO) if the caster is in melee with a hybrid and can cast a touch spell on it, your tank isn't doing his job. I hope to be able to use the lunar memory spell on a player one day. Read the spell and see why (hint: it only works on afflicted lycanthropes).
After spells, we get some magic items of various levels of usefulness. The amulet of control is great and addresses one of my complaints with lycanthropes: both natural and afflicted lycanthropes are CR +1 templates, but afflicted lycanthropes have to burn a full round action to change shape, and might not even change at all if they fail their con check (wasting the full round), making shifting from human to hybrid form actually a very bad tactical choice once combat has started. The amulet makes an afflicted wererat almost as much of a threat as a natural wererat (minus the chance of infection) by shortening the time it takes to transform. I also like that it specifies that it doesn't help afflicted lycanthropes resume human form during the full moon. Overall great item. On the other hand, the next item, Filth Fever Flail, needs a little work. The price/cost does not match up, and the 1d3 days onset time for the disease makes it less useful in combat (I would like to see it work more like contagion with immediate onset). The hand of inglory seemed really weird at first, but then I read the Drinala Greentail NPC...more on that in a moment. Last is roachmusk, very cool concept, but it's technically alchemical, not magical.
Next up are a group of four stat blocks for lesser npc wererats. The stat blocks themselves are fantastic, giving separate blocks for human, hybrid, and (unlike Paizo's bestiary) animal forms. These make it worth the money to me right there. There are some sample npc personalities in this section, but nothing major. The personalities don't seem to have anything connecting them into a "tribe," so if I were looking to use this to populate the sewers of a city with an underground culture, I would probably find the sample NPCs lacking in that respect. If you want to change these over to the crew of a wererat infested ship, each of them uses one of the sewer specific feats, so just change that to something like Sea Legs, and switch out a skill for profession sailor (I traded out intimidate, bluff, ride, and craft alchemy, in order of the stat blocks given). Other sewer-friendly skills like acrobatics, climb, and swim, are also ship friendly, so the stat blocks required almost no modification. The stat blocks put skills in different places from Paizo, and that took some getting used to, but it didn't really slow things down.
Last there are three "personas," very thoroughly detailed NPCs. I only used one of these characters for a stat block (McAdi the halfling rogue, who I used as the captain. I really wanted to use one of the characters from "Villainous Pirates" as captain, but I finally went with a ship fullllll of wererats.), and am saving the others for on land. Drinalla Greentail, the one handed witch monk bent on using the afore mentioned hands of inglory as a tool of her personal revenge, is awesome with a fantastic story, and I am thinking about using her as a plot point in the future. I would love to see how a paladin interacts with her. I'd actually like to see an official adventure featuring her, just reading her story makes me want to watch a certain monastery fall and burn. If I use her, I will certainly run her as a redeemable evil and award at least as much XP for helping/redeeming her as killing her. The last persona, a noble widow with a freakishly large pet roach (the main reason she didn't end up on the ship), could also be a lot of fun, though she's not nearly as sympathetic of a character.
Overall, I liked this book. The stat blocks get five stars for usefulness and thoroughness, and go even farther than the core Bestiary in providing everything you need to run these creatures. I'm not sure about some of the other parts. On one hand, it covers a lot of material and seems very thorough in providing at least a few wererat focused options in almost every field imaginable, from custom class features to new magic. On the other hand, it doesn't really establish a cohesive "tribe," and I think it would have been nice to see some more coordination and connected stories between the NPCs. There are also a couple small issues with content, like the filth fever flail. I'm torn on the rating...I really wish drivethrurpg had a 10 point rating system so I could give this 9/10, or a percentage system, so I could say 87.4232179%. Very useful, not quite as good as it could have been, but overall, I do recommend it.