As an NTL cable modem customer in the LU7 region practically since day one of it being available here I've got to admit they've ben pretty good.
Apart from the ocassional DNS and cache/proxy issues i've not had more than a few minutes of downtime in the past couple of years.
However my first 18 months were pretty hit and miss, I think due to a combination of a poor install (we've got 3 STB's and a SACM - the installers used a single line for them*) and the original 3com CMX modem (which eventually burned out), however since 3com exchanged it for a Tailfin** we've barely had a hiccup.
From what i've seen the difference in competence and CS between now and 3-5 years ago is amazing, at least in our area, yes there are still hiccups and the odd problem but they've improved massively.
Call waiting times are down (gone are the days I would spend 45 minutes in a queue to talk to tech support), the techs seem to know a bit more, the engineers seem to have improved and the price has stayed the same, but the speed has gone up massively
Both my brother-in-law, his sister and his brother also have NTL cable in my area and they've not any more problems than I in the past couple of years
*And we only recently found out that at somepoint an engineer had left a forward path attenuator in back to front (which explained why our STB's would lose interactive and burn out after a while).
**As the original modem was bought by us, we were told in 2002 when it died that NTL would charge us for the callout (user owned equipment at fault), so 3com swapped it and I just rang to get it registered (something I don't think you can do now).