It's an aid to safety - I leave it switched on. If I was to be out on a track having fun, then I'd switch it off (or as mentioned perhaps when starting in snow). However even if it is switch it off, it isn't completely disabled and will still try to interfere with any lift-off oversteer cornering antics.
Of course it won't over-ride the laws of physics, but it does help keep the car better balanced when the roads are a bit slippery, or you have to make a sudden steering input to avoid a collision. I'd say that a modern GTI would keep you out of trouble in the majority of unexpected 'moments' even without ESP. Having said that, in a slide every little helps - we all make mistakes after all.