In real life the systems are widely different. On my PT Cruiser the TCS just backs off the power, so you're a lot better off turning it off and working out the best accelerator position for yourself.
Whereas, at the other extreme, an F1 car has software which arguably gives you the best possible traction under any given conditions. Even then some are better than others, look at Renault at the start of a race.
I find in GT4 that TCS is implemnted the same for every car, which of course is not the same as real life. Personaly I like to drive without it and use the analogue stick to control the power. My father races a Mini Cooper S (back to real life now) and I know he always turns off TCS and ASC when sprinting.
ASC in real life seems quite nice on the mini and I've heard it's brilliant on the Skyliners, but in GT4 I don't like it at all! I find the feel of the car is ruined and my speed through corners is reduced. It seems to make the car harder to drive and predict.
Obviously this is my personal feeling and it may be that if I used the ACS more I'd get used to it, but I'm not convinced!