The advanced bestiary was originally done for 3.0 and it was a solid book then and is perhaps even more so now. Here's why.
Useful templates: The templates in this book vary from interesting to very interesting so much so that entire campaigns could be built around the application of the template. Examples include how various outsiders infuse or manipulate creatures to change them and spread their influence. You could be encountering demons at early levels through their possessed thralls or even qlippoth. Other useful templates are element infused to populate your planes and others that you could plan a world around. The templates that aren't so grand in scope are at least quite fun.
Art: the art is really top notch and equal to the quality Paizo churns out with its books at least. Some of them have backgrounds which I'm not a fan of but that is a minor complaint. You'll love the art.
Sample creatures: the sample creatures included have a good deal of thought put into them and span a wide range of CRs. My only two caveats here is that many base creatures are used more than once even though multiple source bestiaries are cited (really shouldn't happen) and there are a lot of undead templates. They are good templates but I don't use a ton of undead.
Are there any really cons to the book? I don't think so. Some of the templates are complicated but worth the effort. There are a lot of side boxes that help you integrate and flesh out your new creatures to make them feel as though they belong instead of just a mish-mash obvious template so that is a plus.
It's a great monster book! Full of templates that spawn great ideas for adventures. I highly recommend it.
The Tome of Horrors 4 is a decent book for any collection. I'll go over some pros and cons and then overall.
1. There are a lot of decent monsters here of a variety of challenge ratings and types which are easily usable in any campaign (not too campaign specific). Oozes, plants, constructs, they are all here in fairly equal amounts.
2. They have nice color pictures of every monster. This is a big plus for me as I scan and make counters of the monsters. This one fact has kept me from buying other monster books from other publishers and this one. Color is king for me. The art was all done by the same artist who has a style reminiscent of the planescape series though I'm not sure if it is the same person. It ranges from very good to somewhat stylized for certain tastes.
Cons (and these are somewhat picky)
1. Whoever designed these monsters was very centered on spiders, snakes, scorpions, rats and monkeys. Especially the first two. These creatures are always frightening on a base level but it is too easy to make another spider, snake or variation without it starting to get a little boring. Look for new monster archetypes other than spider and snake.
2. No animal companions rules. We get some regular animals and dinosaurs but nothing is provided for animal companions which is kind of sad since that is always done in all the bestiaries. It could have been a table in the back (if I missed this I apologize).
3. More text. Some of the pages have a lot of space that could have been filled up with descriptive text about the monsters. Even if it was just fluff to explain how the monster feels and hunts it is space that could have been filled. I'm always wanting to read a bit more about the monsters.
Overall: I'm glad I got this book. It has a lot of what I like and while not all the monsters were hits with me, the vast majority were by like 95%. I recommend it to anyone who likes monsters.
I was a real fan of the Pact Magic mechanic when it came out in 3.5 so I was tickled to see it done in PF. Radiance House has done a very nice job with making a good adaptation and making it their own.
There are some differences so be aware.
First, the entities you bind to yourself are no longer called vestiges but spirits. Not sure why. Second, the goetia-based spirits from the original TOM are not to be found here which was the only disappointing thing for me. You have lots of spirits but they are all like they were once living creatures instead of the extreme creepy weirdness from the original TOM. I might have to convert the original vestiges to this system as I really liked them.
Anyway, there is a lot to sink your teeth into here. Volume 2 is worth getting as well as it has options for classes found outside the core book and even utilizes character background rules from the Ultimate Campaign book. I'm looking forward to rolling an occultist (the new binder class) and seeing how they play. Looks like lots of fun!