I've been tempted to wade into this thread for a while. Out of the dozen or so cars I've owned, maybe 2 (including the current one) have had ABS, and only the current one had any form of stability/traction control. I've generally been in the "less is better" camp in terms of electronic aids - my first car with ABS (405 Mi-16) had a very early system which was often overly intrusive which has perhaps influenced my thinking. I now have a 6 year old BMW with a much more recent system, though still no longer state of the art. On Saturday I had the opportunity (on an airfield) to try a number of exercises involving hard/emergency braking and avoidance actions, or simulated emergency lane changes. For most of the exercises on a hot day with sticky tarmac it was quite hard to provoke the car to a state where the electronics actually intervened, however it was very apparent on the higher speed lane change (aiming for a small gap between cones at 50ish mph) that the ESP made a major difference to the stability of the car and ability to keep it in a straight line - particularly if you were rough with the steering input. I think the point at which it did intervene was sufficiently outside the usual road driving comfort zone that it's unlikely people would rely on it to let them go faster - and to be honest, if they're going that fast they're going to crash sooner or later anyway. It might be interesting to repeat those exercises on a cold, wet day when I'm sure the electronics would be working much harder.
I'd like to think when I'm making progress I'm concentrating hard on what I'm doing, therefore I'm hopefully unlikely to put myself in a position where I would need to rely on electronic aids to get me out of trouble. However after a long days drive I often trundle home along the motorway - and it's in these situations I'd accept I may not be paying as much attention as I could to matters. It's this sort of situation where ESP could be a lifesaver should an emergency evasion be needed. Yes, I'd like to thing I'd be observant enough to spot a developing situation early, but we're all fallible.
As others have said, whether a car has ESP (or indeed ABS) would make little difference to the way I would drive it, but I now understand better where it would be useful. Would I pay extra for it on a car? Depends how much it costs, but on balance probably not - and perhaps this is a common view which may well be why the EU is mandating it. Will it reduce accidents? Yes, it probably will.
Oh, and yes, I'm glad there's a proper "off" switch so in suitable circumstances (like an airfield/track) I can be a hooligan