Had NTL, then Virgin - switched from Virgin when they told us that staying with them after moving house (which I was quite prepared to do despite not being in a cabled area) would entail closing our account, starting a new one, and thus losing all our email addresses etc (the main reason I wanted to stay with them in the first place). Virgin's cable service was great - I particularly liked the catchup TV and the TV on demand features.
BT are ok - the stuff works, and is easy to set up. Customer service (when things go wrong) is pretty poor, in my experience, delivered by an overseas call centre for the most part who seem extremely anxious to get you off the phone. (NTL and Virgin's service was much better). BT are also very inflexible - I wanted to change our activated line, and was unable to without cancelling our contract (and paying the cancellation fee) and reconnecting. The upside is that the homehub is a nice piece of kit, is easy to set up, and the service is reliable.