We were with Telewest originally (and lost our connection the first night when a car hit the cabinet - hardly their fault) but went to Sky broadband when switching from dial-up (The company paid, and would only pay for the cheapest service.) Unfortunately, the 2Mb service often didn't connect at all, let alone exceed 0.5Mb, so, after being told Openreach would charge us £120 to check the connection we switched back to Virgin.
Stayed with Virgin ever since. A few problems with our original modem which needed resetting about once a fortnight, and noticeable throttling when W8 appeared on Dreamspark but since the new modem and 30Mb upgrade no problems, and if they have throttled us I haven't noticed.