Its is a tough one to call, & i think NTL have pretty much covered themselves with the "upto" 10mb.
Personally i would find anything below 3mb during peak times to be unacceptable, & perhaps around the 5/6mbps during off peak useage to be below unacceptable levels. I would expect my connection though to sit above these speeds the majority of time...
As RDHW as said it will be very difficult to continually get your full 10mb 24/7 & especially if the caps are enforced people wont be maxing there connections continually, so people are going to have to use common sense about this, but i feel this is going to be where it all falls over, as for some reason, internet / speed & common sense never tend to go hand in hand...
Regards
Barfly