The Monk, Ninja, and Spiritualist archetypes are the standouts here. Monk gets a style-switching elemental setup that’s pretty cool, Ninja gets an archetype (rare in itself, and you now have a non-Tien flavor option), and Spiritualist gets the largest skill bonuses I think I’ve seen, giving them a flavorful skill monkey option. The Ninja is penalized too much for not wearing a mask, which is rough with UC Rogue overshadowing it. The book has nice talents for various classes (Investigators can whip up extracts at combat-viable speeds, and Vigilantes get sizable knowledge bonuses) plus bloodline-esque options (Wizards get a Divination subschool that is powerful without being must-have, good for replacing the base school with, and Psychics get a discipline with discounted reach metamagic). Overall, enough stuff that I’ll solidly consider on the relevant classes for me to give a five-star rating.
One cool thing worth noting is the minimalist options. Medium and Occultist both get archetypes that don’t replace much, but subtly change things about the class. Neither is something I’d probably use much, but it is something I’d like seeing more of.
The last reviewer seems to have misunderstood the power of a trait. The only thing I might consider overpowered is the Spiritualist’s large untyped skill bonuses.
I feel a little guilty handing out a five star review based largely on a single archetype, but you can now play an effective poisoner, and I feel like that's a pretty big deal. Not just "I have a special, unique poison I can apply to my blade", but whipping up a broad range of injury and ingested poisons with DCs that keep pace with 6/9 casters. There's a way to deal with immunities, and you can even customize your poisons with effects (either good or bad!) that apply even on a successful save. And it does it without hurting the effectiveness of the base class. This is a book that has accomplished its mission.
EDIT: Misread the talent. You get a different effect that replaces the base effect, so you're dealing with the immunity but not bypassing it. Since you can still apply your rider effects (admittedly burning through inspiration to do so), the overall review stands.
I feel less guilty about handing out that five star review since the book has some other cool stuff in it. Interesting poisons (need to fake being sick, or maybe just dissolve somebody?), awesome hexes, neat racial options, and some cool new spells.
Uh, don't expect much from the potions side of this thing, though. Potions are already kind of pricey for what they do; I don't want to spend feats to get effects that work when I spend even more making them.
Dedicated shifter class. Even if you don't like the class, it still means there's now a class that will receive exclusively shifting-focused archetypes.
Feats. Dust the bad ones aside, and enjoy rage totem powers on non-Barbarians, wildblooded bloodlines on non-Sorcerers, some nice options for Shifter, and more.
Archetypes. So many great ones! Highlights include a venomous Brawler, a Monk with Kineticist powers, a Kineticist that uses all the elements based on their surroundings, and a Shifter that's an ooze. Animal companions and familiars get in on the action too!
Rules. Lots of cool foraging rules, and much more detailed weather rules. Plus, rules for salvaging magic items and the like when you're leagues from a handy marketplace.
Price. If you grab the PDF, it's just ten bucks. Seriously, go for it.
One of the big complaints with witches has been that the patrons are very bland and don't actually do much. This book tackles the problem head-on, allowing you to get more bang for your buck from various patrons. That's right, patron-specific mechanics are here at last! They're pretty well balanced to not add too much power to a full caster, while also not hurting too much to take.
Subraces are always awesome, even if the changeling subraces are a little clustered in their stats (which don't always match the flavor). The Awakened Hag Heritage feat is an excellent addition to Changelings, too; both flavorful and mechanically useful. Changeling covens are tricky to pull off in a party, but very rewarding.
More curse spells are fun, and Knell of the Depths is enough to justify a Skull & Shackles witch by itself. Curse of Dragonflies also gets a mention for allowing the caster to do something very helpful- force a flier to land in mid-combat. The fact that it's also a permanent curse is gravy on top! Pick it up, and your party's melee combatants will be grateful. In addition to the spells, there are occult rituals. Notably, a "you and your descendants" curse (with options that work even if you're pretty sure they won't have kids), and cursing somebody with a pugwampi.
The new archetypes for witch pair excellently with the new patron rules, and are quite flavorful, with useful tools. Other classes get nice archetypes as well, like a spiritualist bound to an evil and independent-thinking spirit, and a bloodrager that is a solid addition to many natural attack builds.
Every book has some things that don't shine as much. The Malice Binder is stretched too thin by using charisma for DCs on a martial class that uses Int for everything else. Curses and witches don't usually come up enough for me to take archetypes centered around them. (Then again, I didn't get very far in Reign of Winter.)
This book provides a solid improvement to a class usable across nearly any archetype, several good archetypes, good spells, and lots of expansion to an existing race. Well worth a purchase. While I loved the broad coverage of Blood of Beasts, Blood of the Coven shows that an in-depth treatment is good too!
It's nice to have the fiendish planes fleshed out in one handy book, and (if you're grabbing the PDF) for less than the cost of the much smaller source material books. There are a bunch of cool new things you can do, like become a true werewolf or a worm that walks!
The reason it's not five stars is because it's very heavily weighted towards (in order) demons, devils, and daemons. If you're interested in other fiends, there's not a lot of mechanics that work with them, and where they're specific, they're purposefully weaker.