I had an unpleasant time with NTL whilst in the UK - we had no phone service for a couple of months when we first signed up and their customer service was really appalling. When we finally left, we moved to Ireland and gave them a forwarding address. When I didn't get word of a final bill, I called them up and paid the final sum of £10.40 over the phone. A few weeks later I discovered that they had sold the debt on anyway to a debt collection agency who were vigorously pursuing us for the bill and related charges. I couldn't believe that they hadn't bothered to use our forwarding address and had sold on such a piddling debt so quickly - and they hadn't bothered to resolve it when I called to pay the final bill.
Needless to say, I wouldn't touch them. However I know of some people who have been happy with them.