I thought it was excellent. The best d20 overview of a feudal middle-ages style country. Very useful for a Hyboria campaign but easily mined for general RPG use as well. I've used it extensively in conjunction with Western Europe: A Magical Medieval Society for my Kalamar campaign (which I run using the Conan RPG rather than D&D).
Sections on provinces and how "average" characters could be built with respect to skill selection, Codes of Honor, classes, etc. Sections on government, military structure, prominent NPCs, a substantial gazeteer on the country itself and sample (non-named) NPCs. I'd consider it a must-have for a campaign set in Aquilonia and it dovetails nicely with Across the Thunder River (which I ordered immediately after reading Aquilonia).