Sorry to hear about your crash steveb. Glad you were all ok.
I went on the new RS4 driving experience at Silverstone in November and some of the elements were designed to show you how effective ESP can be. I have read the magazines, I have tried messing about on roundabouts / hairpins etc in my 3.2Q but it was only on the circuit that I got to appreciate what it really can do.
Whilst the RS4 has the option of completely turning off the ESP rather than in the A3 where it only disables certain elements (and switches back on if you hit the brakes, even if you think it is off, apparently), it really does make a difference when the car is not going where it should. The ESP works on the basis that you want to go where the front wheels are pointing, and appears to be able to help in most normal driving situations, but in extreme cases, it is not always going to be able to correct such excessive differences between actual travel and desired in such a short time period.
To show how it can work on the course, one section was to drive at 60mph towards a wet sharp bend, realise you are going too quick, back off, turn in and then touch the brakes as most people would in a corner as the car was understeering. This leads to the back swinging out...the goal is try to correct it all without ending up on the grass or in the gravel. We were a pretty amateur bunch, especially compared to the instructors, but I think all 22 of us managed it with ESP on, but the comedy started when it was switched off - some serious spins and sideways grass slides from most of us.
I do think it really does help, but it is an aid, not a failsafe answer to extreme situations.
Oh, and the RS4 was truly amazing. From the sound, the unending power (140mph in 4th) to the little things like electrically powered bolsters in the seats which squeeze you in if you press the S button on the wheel. If they do it again, you have to try it.