Well i do have just finished with setting up FastDL for all my SRCDS servers that i'm hosting!
FastDL is ofcourse Fast Download most of the resources needed from server to client as example ( GMOD ) are on the SteamWorkshop where people get their custom content for servers.
What makes FastDL usefull is that its downloading the resources from your HTTP server ( WITH PROPER SETUP ) will be much faster as Steam and only download the needed custom content and not all including workshops ect!
Most important to have an FastDL good functional is that the files are compiled in most time BZIP fileformat.
Content that allot of people would propably already have like maps ect. I would recommend to not FastDL this on your server becouse an big FastDL with allot content isn't fast anymore.