I spent the best part of half a day testing on a variety of surfaces & surface conditions ESP on/ESP off
basically it won't throw you into a ditch when it's on, it won't cut all your power & leave you in the path of a truck.
what it will do is prevent you from fishtailling out of control (but you can still get a whole lot of lift off over steer & pull it back in)
you can still get the wheels to chirp, it's not 'bad' TCS that cuts all power
having it on isn't a get out of jail free card, but it certainly raises the bar.
There's a thread on here somewhere where a
Mk1 Leon owner was caught out on his "private road" when he went round a corner at about 60, and ESP off slammed into the ditch & field beyond.
even he admitted that he thought he was driving god & didn't need it.... right up until the car went over.