Summary: Indispensible for a planar campaign, best focused on the Inner Planes, or on Axis. Equivalent in usefulness to the "Player's Guide" versions of 2e Planescape books.
Allusions to medieval religion and cosmology sprinkled throughout make excellent ways to tie in real-world mythologies. A favorite moment of mine was Melek Taus as a missing Archangel of the Heptad. Dante makes numerous appearances. The production values are good; the text is searchable, the pdf renders well, and the artwork is excellent.
The book lacks "current events" descriptions, so GMs will need their own plots (or try module J5!). There are planar mysteries and tensions, described plainly, but of long standing; no current upheavals. An interesting exception is a half page on the contested control of Aroden's domain near Axis. There is no Sigil; if a GM would like a Sigil-surrogate and a handy adventure series, Axis (or Shadow Absalom) could do well.
Many people and things come from "elsewhere," and seem to know but remain silent on where, a trick used a bit often for me -- but this permits uncertainty on the nature of gods. Some gods may be dependent on belief, but some seem to predate mortals and envision existence after them. (The Axiomite Godmind is neat!) Their conflicts seem purposeful rather than self-aggrandizing. This I like very much.
One thing seriously bugged me. "Atheists" from Golarion are apparently so foul that regardless of alignment Pharasma sentences them to be buried in crypts after death: they are "self-damned," "poisoned,” without hope of rebirth and occasionally fed to Groetus, daemon-like. Why? What corruption in atheism is so pernicious that it would survive reincarnation?
While I would like an answer to that, I don't want to end on that note. As a GM, I will retcon it out, as I did when the sentiment appeared in Guide to Hell. Overall, I liked the book. It has inspired gaming ideas already, which is the primary reason I buy any RPG setting material.