The Kingdoms Of Kalamar Campaing Setting is a very detailed book that skips completly over the "crunch" that seems to be a prequisite for most D&D book these days and instead get down to the serious buisiness of discribing the world.
Although many of the human realms are inspired by real world examples (Rome etc.) they still manage to portray a very different world. In the world of Tellene, humans dominate the world but are given heaps more flavour than the standard fantasy humans. Kingdoms of Kalamar uses human sub-races that each have individual appearances and culture.
The section on language further enhances this and although I may never find a practical use for them (apart from naming my characters of course) they add so much to the setting, showing that there is actual structure behind the some say bizarre place names of the setting.
The gods of the setting are, despite whatother reviewers have said, original and their depth is amazing. With information on the organisation of the churches, sacrafices and holy places, it is much better done than any other setting I have seen.
The minor sections of the book, like the armies of Kalamar, laws and section on Tellenes constilations are not entierly nesesary but they add even more detail to the setting already displayed.
Kalamar presents as a low magic campaign world but can be played at any level. One of the things I like most about Kalamar is it's versatility and the magic of the world is a prime example. There could easily be no magic or flying cities it's up to you. This differentiates this setting from those like the Forgotten Realms that are stuck at certain levels.
A mature look at slavery is one of the best things I like about the setting as it is fully integrated and can be used as a moral device for my players.
All in all the Kingdoms of Kalamar Campaign Setting is a very good buy for any game( because the D&D stuff in it is limited to some horse breeds, a few magic items and a NPC index) or indeed just as a source of ideas and material.