An example of how ESP can be bad;
A few months back I had access to a huge carpark where I started dabbling in provoked lift off over steer. I was able to get some decent angles before the ESP quite voilently kicked in and almost snapped control out of hands. In some ways it seemed to unsettle the car, although that was probably just my interpretation - it certainly stopped the slide. Doing the same with ESP off was far more fun and I never came close to spinning the car. That's not to say I woudn't have, in a genuine non-provoked accident invoking situation though!
To clarify some of the above - yes, ESP is OFF when the warning triangle is illuminated. I often think it's mostly placebo, but other times I'm convinced it makes the car faster, most importantly coming out of tight corners where the steering angle seems to have some electronic hold over the power available. Probably only a programmer involved in the ESP software could confirm if this is true.