Cliff Notes: Go out and buy one of these if you're a strobist!
I recently acquired the SU-800, as I was beginning to discover the strength, convenience, and versatility behind Nikon's CLS system. The SU-800 essentially act's as a commander unit for CLS compatible Nikon speedlights. Most folks who are picking up the SU-800 are the Nikon shooters with the pro bodies that don't have a built-in flash on the camera bodies, or are interested in doing macro photography (the SU-800 is included in the close-up/macro flash kit). I purchased the SU-800 individually for $250 through b&h photo/video.
I already have several SB-800's that have the commander functionality, in addition to my D300 which also supports and acts as a CLS commander. So why did I pickup the SU-800? Here are my reasons: