If you track the bike a lot, get it. Besides mag wheels, a slipper clutch was one of the best things I put on my bike(Track 996S, with a bad ass engine from Ducshop). They all work good(Simple concept). I have a STM EVO and have heard good things about EVR.
I do not have one on my street bike (S4RS), but I am also not dumping three gears and going from 150 to 40 MPH on a routine basis...

..I don't think you need one for the street