My gaming crew tested out the Pathfinder Beta rules when they first came out, and we were all impressed with how user-friendly it all was. Skip ahead about a year and, with the advent of the Pathfinder Bestiary, Paizo has continued their tradition of support for D&D 3.5 enthusiasts while, again, making things just that much easier for the DM to handle.
The Bestiary is a beautiful hardcover volume of 320+ pages (with Wayne Reynolds art on the cover, by the by) and contains 350+ monsters, from Aboleth to Zombie and then some. Here are my thoughts:
The Good: First and foremost, bang for your buck. The production quality of this book is exceptional, from the sturdy binding to the pages, which are decorated with arcane, gothic whirls and swirls throughout. The paper is of good quality, and everything is in full color - everything. Additionally, every monster is fully illustrated, and 90% of the art is of excellent quality. Paizo obviously spared no expense in printing this book and getting it to fans.
Now, for the contents. There are some innovations that old-school players may have to get used to. The Bestiary uses a 3-symbol system (explained at the beginning) to define the monsters: for example, each monster, after its name, has three symbols - one denotes its kind (aberration, undead, etc.), another its native terrain (urban, aquatic, dungeon, etc.), and another its native climate (cold, extraplanar, etc.). Additionally, each creature, in addition to Challenge Rating, has listed its XP awards - no more flipping pages in a DMG to figure out how much XP to give to your players. Additionally, each creature is fully statted on one page, with no space wasted. In some cases, there are 2 related creatures to a page (such as Bear and Dire Bear), but by and large each creature gets its own page (or, if the creature is complicated, like a Vampire, 2+). There are no separate sections for Swarms or Vermin, or to separate regular and dire animals; spiders and spider swarms are listed together, for instance, under Spider, and boars and dire boars under Boar.
Something else that's great - Appendices. The Monster Creation system offered in the book is comprehensive and detailed and seems able to take care of any contingency - you can tailor any creature you make to a particular CR without any problems, see how much XP and gp it should generate, and so on. Same with monster advancement and alteration: want to change the listed creature's size, for instance, but aren't sure how that would affect the stats? There's an app (or in this case, a table) for that. There's also a section on universal monster rules so to avoid stat block clutter, and another on traits of creature types, and even another on monster feats.
The Bad: Not much, but a couple things worth mentioning. First, when viewed critically, this is essentially the D&D 3.5 Core Monster Manual, just dressed up a bit. Certainly its very nice and, I think, worth the money, but this is probably only going to be useful to you if you are planning on writing up your own Pathfinder Campaign, since most of the monsters in Paizo's pre-written campaigns are already statted out for you in those products. Second, the book could have set itself apart by creating a few new signature monsters (of which there are precious few). Third, I was disappointed that the guide paid short-shrift to using monsters as PCs; while not outright discouraging it (and providing PC guidelines for creatures like Goblins, Tieflings and Drow, for instance), it doesn't really encourage monsters as PCs outside of the basic races in the Pathfinder CRB. For some, this may be a minor nuisance.
Overall, I have to say that the Pathfinder Bestiary is money well spent, especially for those of us who refuse to convert to 4th Edition.