I always leave it on for road driving but prefer it off for track. When at bedford I found it intrusive coming out of the tight bends before the long back straight, tiny bit of wheel spin with it turned off but it was definitely faster.
Then at the 'ring I turned it off, I'd been sideways with it on and didn't think it really did anything. Then the next lap went sideways on the same corner with it off and caught it just as well - think I was quicker than it! Also, the karussel at the 'ring does crazy things to the esp and just puts the brakes on even though there is no wheel spin or sideways action!!