This is the first truly 'new' supplement released for the Greyhawk line (the others were mostly rehashes of old material). The title is somewhat misleading, as only some 30 pages are devoted to the Brotherhood. An equal amount is also devoted to Hepmonaland, and another dozen or so to the Amedio Jungle.
Overall, it is of very good quality, providing a good general overview of each area's history, internal structure, and long-term goals providing the needed structure for the Brotherhood, but without overwhelming detail. Quick stats and personalities for major NPCs are given, along with a gazetteer of all major areas. In addition, the SB's character is fleshed-out with details of culture, dress, and even a Suel-English dictionary.
Hepmonaland seems slightly under-detailed, as it is made up of 'city-states' that actually cover vast territories. A greater density (ie more locations) would seem more realistic. One area of interest that has often been neglected in Greyhawk supplements is culture. 'The Scarlet Brotherhood' does an excellent job of fleshing out the SB, the Olman, and the Touv (a new race) with customs, dress, deities, and even language.
The map provided has a nice style to it, and is clear to read (although swamps and deserts could be more clearly delineated). It is, however, somewhat delicate - the size of the maps would have been perfect for a cardstock insert such as found in the Player's Guide to Greyhawk.
The artwork is probably the weakest part - most illustrations (a good idea - illustrating the content of the text, rather than random decoration) is sketchy and rushed-looking. Overall, I think this supplement is well worth the cost and I give it 4 out of 5 stars.