The bulk of Advanced Class Origins is made up of two page spreads for each of the ten new hybrid classes found in the Advanced Class Guide. Roughly one page is devoted to a discussion of common locations from where each hybrid class typically comes from, and the second page goes to mechanical options. The book then goes into new magic items, spells, and feats.
The flavor information is pretty much what a fan of Golarion would expect. Arcanists are from high magic societies, such as Absalom, Nex, and Geb; while skalds are from the Lands of the Linnorn Kings. I personally would have preferred more details on the less obvious choices. For example, skalds are apparently popular in the Taldor’s army, which is strange for the “empire in decline” to encourage raging warriors, and I would have liked to learn more about.
The meat of this book is the mechanical character options, which are to taste. For example, I like both the mechanics and flavor of the black blood bloodrager bloodline, and the seafaring dragon skald. However, some of the archetypes are for evil characters only, or are for very circumstantial builds. I do appreciate the callbacks to prior options, such as giving swashbucklers a whirling dervish archetype, and including scalykind and void blessings for warpriests. Each class spread has regional traits listed, some which are outrageously specific. For example, Lichblood can only be taken by characters from Belkzen with the undead bloodrager bloodline, and gives a +2 on Diplomacy and Intimidate checks only against orcs from Belkzen.
The book includes feats, magic items, and spells, of which the feats especially should have been in the Advanced Class Guide itself. Not just the infamous Fencing Grace (Dexterity to damage with the rapier), but Expanded Spell Kenning (allowing skalds to access the druid and witch spell list) and Pack Tactics (treating animal companions has fellow possessors of a teamwork feat).
Advanced Class Origins is recommended for players interested in the hybrid classes found in the Advanced Class Guide who are looking for a few additional mechanical options.