I am with Virgin and after we finally got a new cable laid to the house it is very good. I just upgraded to 20mb and download at 1850k which I think is pretty good. Was downloading at 470k on the 4mb service before.
The fact they throttle you if you go over a certain amount isn't really a problem, in fact I much prefer that to traffic shaping and nearly every isp is introducing something along those lines and this is the best solution. To be fair I can download 5gb an hour if I want so the 16 hours unlimited a day are quite sufficient.
Customer service is ok, it's premium rate but if you have a valid problem they credit back the cost to your bill, I guess thats to deter numptys from abusing it.