This is pretty clearly the best rule supplement to any role-playing game that I have ever played. The bulk of my role-playing experience has been with D&D, but I have played since the Blue Box, so I feel like I have seen a lot of supplements for the game. Typically when a supplement is filled with rule options, only some of them are interesting. There is not a foot put wrong in this supplement and as a DM I have slowly skimmed it and read different parts as I became interested. I have slowly started to treat the entire supplement as core rules.
I am not a simple Pathfinder fanboy. I looked at for instance the Mythic Adventure Guide and the Gamemastery Guide and I do not see myself ever picking up either of those books as they have numerous problems, but the material here is all excellent.
The supplemental rules - which include traits, hero points and additional combat maneuvers are so good, that everyone should use them. In particular, traits start immediately making the players think about their back story, while allotting what amounts to only the equivalent of about 2/3rds of a feat
The six new base classes are likewise excellent. The Witch is so good, that I merely had to tell one player about it and he decide to be a witch. D&D has over and over again tried to do witches and failed. Here ir is done exceptionally well. The additional spells are very good, but not quite as good as Ultimate Magic, but still very good.
The addition of the concept of archetype for the core classes is also excellent. These allow you to take the existing classes and swap out powers to create variant versions of the traditional classes. Swapping out the existing classes is very well done and allows customization of your character in ways that are intelligent, balanced and exciting. The chapters on Races, Feats, Equipment and Magic Items are of a similarly high quality. All told, the whole work is very very good. It is a must buy for any Pathfinder group. The whole work is available on the Pathfinder website as part of the Open Game license and you can check it out before buying. There are also new prestige classes here. I am not a great fan of the Prestige class concept and I think what Pathfinder is doing in creating new base classes and new ways to customize classes is the right way to go because it allows you to play the character you imagine from the start. But for those who are interested, these prestige classes are all very well done.
Everything is thought through in great detail and should be treated as core rules, because there is really nothing here that does not make the game more interesting and produces a sense of individuality to the character creation process. By combining different combinations together in different ways,anyone can create a character that is truly unique. Also, the book is a great resource for GMs. The character options here are so numerous and interesting that NPCs can turn up and you can guarantee that you will completely surprise your players.