A remarkable work. While listed as d20, it's very suitable for all gaming systems, even Sci-Fi games (if you ignore references to magic). Very good also for non-gamers who wish to create secondary worlds, again ignoring certain references.
On the bad side, you need to understand that the lower quality of exterior and interior art is not indicative of the product's actual content.
For gaming, I especially liked the listing of magical creatures that, while not tough enough to count as monsters, add flavour and different kinds of challenges for a gaming session.