This is a solid addition to 2E and well worth adding to your library, although uneven or even disappointing in places. It feels something like a mix of 1E Advanced Player Guide and Advanced Class guide with less ancestries and classes but I like that: nice to have a bit of both rather than to get a bunch of one while waiting 6-12 months for the other.

Pros:

Witch, Oracle, and Swashbuckler are well designed with clever rethinking of mechanics that adds new dimensions to the classes and definitely improves playability with respect to other classes. I especially liked the witch patrons that could make your witch more like a prophet or a fate-weaver while still providing the usual curse, night and wild options for your classic scary witch. The oracle curses are much more interesting- and much more of plusses with minuses than the old version. And swashbuckler seems both quite playable and fun.

Versatile heritages are a great re-think, one of the best parts of the guide. While less potent at low level, the ability to add tiefling to any race, plus the new versatile heritages and the promise of more, greatly expands the range of character concepts.

Archetypes are nicely fleshed out. While the system was in the core rules, they don't really shine until here. Many will appeal only to a specific concept but can have their uses while others are significantly useful for those focusing on combat in particular. You will recognize many names from prestige classes of yore. While not, in general, as potent as an old prestige class, the move to archetypes is both more graceful and more manageable for all- players, refs and game designers. Many can be taken at lower levels and others at higher levels.

While familiars only get used so often in my games (more so by me as a player :) the extra abilities, feats and specific familiars are great. I especially like that it is both practical and clearly explained how to get an imp or faerie dragon.

Feats and spells are nice, mostly as they relate to new classes and archetypes. For existing classes, probably less useful but there are exceptions.

More middling:

Investigator seems suited to a limited range of campaign types. I wish it was a little less detective-like and more lore focused, but I think for the right players and campaigns, a good option.

The new races, while definitely a nice addition beyond too human-like variants, are also unlikely to get used much in my campaigns, except maybe catfolk, although they all seem well executed.

The new backgrounds are so-so. They are nice enough and its not like backgrounds are a particularly eye-catching part of the game, although it is a nice mechanic. The rare ones were a bit disappointing to me, but again the real flavor of them is left to the player in character creation so they are solid enough.

The core classes additions were a very mixed bag. Some are quite interesting and others are so narrowly drawn as to appeal to very few players. I'm thinking of you druid, where the additions are not likely to apply to most of the druid orders. In general, core classes deserve another round of additions like the 1E combat, magic and other guides. The current crop of goodies may disappoint many.

Overalll:

A strong guide. Hopefully upcoming Golarion and other guides will continue to flesh out 2E.

For those looking for more ancestries, classes and archetypes, I would certainly start with this guide but note that the Golarion books, both already published and planned, add a fair amount, almost all of which can be used in non-Golarion settings. For example, apparently many of the 1E Advanced Race Guide ancestries will be coming to a Golarion guide early next year.