My experiences:-
Broadband... can't fault it. Downtime has been virtually non-existant and speed has always been what I'm subscribed to (currently 10Mbit). It is more expensive than the competition, but (in my case) it's a case of you get what you pay for. Also, they scored a lot of brownie points with me when they did the upgrades not so long ago. 3 years ago I was paying £35 p/month for 3Mbit... now I'm paying the same amount for 10Mbit. I wouldn't be able to get 10Mbit via ADSL where I am.
Phone service... was fine. Worked 100% of the time. Again, was not as cheap as BT - line rental is comparable but add-ons such as CLID, calling abroad packages (which we use a lot due to my missus being French) are slightly more expensive, but not massively.
DTV - had a few problems with this. Red button never worked ever. Strange problem with channels changing automatically and STB resetting itself everymorning at 1am when the EPG gets updated (why they didn't do this at a more convenient time is beyond me!). Also, our cabinet would be down for a few hours whenever we had a thunder storm... also the door to the cabinet has been left open for years, despite informing NTL of it. Even the engineers who visited said it needed repairing, but they didn't have the materials to do a proper job so it would never get done. Base package was very basic (although I believe this has improved since I left).
CS - sadly this was the pits. The CS was the real reason I left NTL for BT & Sky (except for broadband) 18 months ago. If they set everything up correctly initially and you don't need to change anything then you might be OK. But if you upgrade/downgrade, or if you move house, then you're asking for trouble. Like someone else mentioned, they don't always seem to record stuff correctly so vital details often go missing... like the fact I'd moved house a year ago (despite their excellent engineers having visited to disconnect and reconnect me) but was still having to explain to them that I didn't want to pay for 3 services at 2 different addresses.
Technically, NTL was good for me (and still is for my broadband) but they let themselves down by poor processes and "people-problems".
On the other hand, Sky have treated me very well (they messed up once by subscribing me to the film & sports packages when I had said I only wanted the family pack, but to their credit I phoned them and they rectified the problem the same day, crediting my bank account for the amount they had taken immediately (NTL only ever wanted to credit my account for the next month's billing cycle, but they kept on screwing up so I never got to benefit from the refund
). As a gesture of goodwill, Sky also allowed me to keep the movies and sports channels for free for 3 months... it wasn't much but it was appreciated after not having received anything from NTL for the problems I had encountered.
Oh, and I keep on hearing stories about Sky being poor quality during bad weather... not had this problem personally, and the quality is as good - if not better - than it was with NTL's DTV.
Having now tried Sky, I don't think I'll ever go back to NTL for DTV.
As for BT... not had to call them about anything yet, so can't really comment on them. Service has been 100%, price is OK-ish. No problems with 'em ...yet
One last thing... although it is more convenient dealing with one supplier for 3 products, I always used to get 3 separate bills - with 3 times the amount of the same publicity material each month. Not sure if this has changed since, but it used to annoy me as it made it more complicated to keep track of everything than it should have been. One bill per month on one sheet of paper for all 3 services would have been much more helpful.
Overall, I was severly disappointed with NTL. I love their broadband products, and the DTV & phone were OK for my needs (I don't watch much TV to be honest), but I felt I was constantly being p*ssed on from a great height by their incompetent customer service representatives.
If you do decide to go with NTL, make sure they get everything right the 1st time round. Good luck with whoever you decide to go with