I have been with Telewest/Virgin for years, and have always had a good experience.
Running regular speed checks I always get the rate that I expect, and when its throttled I get the rate that they say it will be throttled to.
I have consistently had about 100% up time except when my modem itself went a little funny, but a couple of resets on that seemed to resolve the issue.
Very happy with the service, currently on a 4MB line, which will be upgraded to 10MB for free... although this is listed as TBC in my area at the moment (Bristol).
Cable seems to certainly be more stable, and I havent ever paid for an amount of bandwidth and got less than that really. Just be aware that the upload is lower than ADSL, but hasnt really been an issue for me.
I have had a few calls to the call center as well (about cable TV), and depends who you speak to, if you get a good person then they can query your devices over the phone and do any updates or resets then and there.
The TV on demand is also good i.e. if you missed Top Gear or something, you can just browse for it and watch it at your leisure.
Another note to make though, if you get them to put cable in your house, get them to pipe it in at the front and put the box there, and run any extra cable yourself, as they can be a bit crude at running the cable sometimes, just make sure you let them know exactly where you want them to go, as coax stapled around you house can look quite unsightly.
I should be getting 4Mb down and 384Kb up, and currently Im getting:
Which is about on the dot.
See here for throttling info:
http://allyours.virginmedia.com/html/internet/traffic.html