Yes, it is. Basics of archetypes, majority of the base classes, more feats, gear, spells, traits (some of this has been reprinted else where) and combat maneuvers.
If there is one pathfinder book everyone should get besides the core rulebook and the bestiary, its the advanced players guide. It is what made pathfinder pathfinder and not just 3.75 dnd. It contains much of my favorite material produced by paizo, and introduces 6 interesting and fun (in my opinion ofcourse) new classes with a host of new options for all sorts of characters.
Bascially, the short answer, (particularly if you like new rules) is yes, its worth it. Yes you should buy it.