They're great. Runout on the prototype unit pictured below was .0003"!
Copy and paste of my review from another site.
Driving the car now is totally different. Noise free operation, and it works as an LSD should. I can turn in earlier and not understeer as with an open diff. Late turn ins, the car goes where I want it to. Just point and gas and the car remains neutral versus having to lift to get it to oversteer to overcome the understeer. I beat the crap out of it. Pounding it over "gators" as you would on track, putting one wheel off, 7,200 RPM clutch dumps, wheel hopping, everything.