I've been using Voipfone since around October 2005, whilst in the early days the service could be a bit flaky, now it is fine and just works. I should say that we do not make many calls but when we need to it works.
Support is also quite good, they do actually get back to you. Usually within a few hours.
BT -> Zen -> F2S -> Bulldog -> Be* -> BT Infinity
Far too many computers, 1 Wife, 3 Maine Coons and too many horses