nffc has summed it up very succinctly, imo. If they get things right first of all, then you're laughing. However, if/when things go wrong, that's when you'll regret having chosen ntl.
Technically, I never had many issues with them - I'm not sure exactly what the uptime percentage would be, but I wouldn't be surprised if it was > 99.5%. However, it only took a relatively simple task such as moving house for it all to go pear-shaped administratively. I've not yet fully ditched NTL, and still have an on-going problem since August 2004!
Saying that, I think that BT aren't much better. I'm still waiting for them to make an appointment to get an engineer round to inspect our socket... hopefully it'll be 22nd Feb (after already waiting several weeks)
Sky, on the other hand, have been exemplary, and NTL would do well, imo, to look at Sky's level of customer service. I haven't looked back since switching to Sky.