This book is what 3.5 should have been. In retrospect 3.5 was where D&D started to go downhill. I suspect that this is when corporate types that had no understanding of role-playing started making core decisions and started alienating their fan base, 3.5 did not really revisit 3rd edition, but instead made a few tweaks to the game that were nor really better or worse, but caused people to have rebuy the books or to work with two slightly different systems. In contrast Pathfinder revisited the rules, reconsider everything and fixed all the problematic stuff. Unlike the disaster of 4th edition, Pathfinder is the true successor to D&D.
The main fix is that the core classes are boosted in power. There is now interesting new stuff for almost every class level and the core classes are as good as Prestige classes. Pathfinder gives a lot of love to the Core classes and quite a bit of flexibility which is only increased by the other supplemental books. That means that the core classes are just very interesting to play.
Everything else has been improved as well. That has been done by revisiting everything and cleaning it up where needed. Some of the highlights are combing The Player's Guide and The Dungeon Master's Guide into one book. The addition of the concepts of the Combat Maneuver Bonus and the Combat Maneuver Defense. This brings all the special actions in combat like bullrushing, overrunning, feinting and sundering under one system of rules. In particular, it simplifies grappling so that these rules are now simple enough that players are going to be tempted to grapple, instead of avoiding it because the rules are such a headache.
Another standout is the overhaul of skills. A number of skills have been rolled together. In particular - search, spot and listen have been rolled into one skill of perception, hide and move silent rolled into stealth and balance, jump and tumble into acrobatics. This is just much better and quicker. Also, a skill being cross classed is no longer such a big deal. Any character, can overtime, become good at any skill. For some classes, certain skills will be easier. First level, also works like all other levels for skills and that is just much better.
There are almost double the number of feats as there were in the 3.5 players guide as well. Magic item creation has been improved and players might do it now. In this book, there is not one change that I have noticed that does not improve the game.
My only beef with the game is that it continues to put too much emphasis on combat. From the start the bulk of the rules for D&D were always about combat because it evolved out of miniatures gaming. In 1st and 2nd edition there was a lot of hand-waving in the rules, so the fact that everything outside of combat was blurry did not matter so much, because everyone had to role play out situations including combat all the time. Once 3rd edition cleaned up combat, some more robust rules for everything else might have been useful, so that the game supported adventures that were less of the traditional dungeon crawl a little but better. The supplements address this a bit, by putting in spells that are really only about role-playing more frequently. Things like the Game Mastery Guide or Mythic Adventures don't fix this because there rule sets are bad.
However, Pathfinder is what 3.5 should have been and people playing any edition of D&D after 2nd edition should switch to Pathfinder because it is doing what 3rd edition did, only better and 4th edition is not worth playing at all.