I want to briefly add my voice to those expressing immense satisfaction with this Bestiary.
To me, one of the key factors separating Paizo's Bestiaries from vintage Monster Manuals is the breadth and range of species represented. Not simply a compendium of monsters, the 5th Bestiary does a particularly good job of covering a range of Neutral and Good beings, who might serve as compatriots or guides or influences on players over the course of an adventure.
Beings such as Androids, Anunnaki, Gray Goo, and Rhu-Chalik commit to the flexibility and range which Pathfinder has steadily earned, bringing the possibility of futuristic and speculative tales to the table.
New and old inclusions such as the Firbolg, the Grim Reaper, Manasaputra, and the Gray demonstrate Pathfinder's inclusion of a wide range of traditional influences.
Creatures such as the Deep Ones and Leng Ghoul (Lovecraft) and the Esoteric Dragons bring the possibilities further into the unknown.
Such efforts, along with recent supplements such as Occult Adventures and campaigns like Iron Gods, show Pathfinder to be a system just hitting its stride.
Still reaching into unknown areas, Paizo is not afraid to stretch its lore into areas carved out for speculative fiction and weird fiction. This is an important investment in the longevity of Pathfinder and the OGL effort it embodies, and the 5th Bestiary is an excellent example of this reach.