So apparently Steve Vai plays with a modded DS-1 which is the Robert Keeley mod..
what exactly does he use this pedal for?
an overdrive?

what's it like on a clean channel?

tell me more about it and stuff..
I would say he uses it to boost his amps Distortion channel for leads and such.

I doubt he leaves the thing on as his main source of distortion.
Fender 60W Super-Sonic Head
Fender 2x12 Cabinet (Vintage 30s)
TC Electronics G Major
Boss Blues Driver BD-2 w/ Keeley mod
MXR 6 Band EQ
BB Preamp
Jim Dunlop Crybaby Wah

American Fender Stratocaster
yeah, he wouldn't use it as the main deal but I wasn't sure if he kind of added the distortion from the pedal with his amps or used it more of a overdrive.