I have been following the Quaife group buy threads with interest as sick of not being able to get any power down in the slightest of damp conditions and if giving it the full treatment dry as well.
Was looking before the above threads started and came across Wavetrac and the seem in the same...
My friends recently fitted one to his BMW and seems to be very happy, don't know how they compare but the price seems pretty good.
Apparently for street use, they offer better grip in low traction conditions, which most of us face every day.
Wavetrac® Differential - A torque biasing...