The Wavetrac is a nice unit for a street car and the price is pretty good. But once you actually have the LSD unit, you need to install it into the case and that involves a bunch of extra work and cost to get it right. Setting up a diff properly is pretty easy, but there are some special tools (a pair of really beefy snap-ring pliers and a low inch-pounds torque wrench to name two) that you need to do it properly.
'03 330i ZHP - Stock-ish
'02 330Ci - Megasquirted, E85'd and tuned by PFTuning.com