Sorry to bump an old topic, but I wanted to add I've been massively impressed with O2.
I've just finished setting up O2 broadband (my first ever ADSL after having used cable for years and years, and moving to a non-virgin area) and the service has been great. I encountered an issue a day after activation - and at 7pm at night a nice helpful guy spent almost an hour diagnosing everything, and was even ringing the relevant BT folk to enquire about an issue noted against the line whilst I was on hold. A similar issue on Virgin would have taken weeks to resolve whilst being passed around.
And as a bonus, as part of the diagnosis they had me connecting through the BT test socket (behind the front plate on the master) where I noticed my speed tests went up from under 10Mbit to almost 17Mbit. A £7
i-plate later to filter out the bell wire (I didn't want to fiddle with wiring as this place is rented and nearly new) and I'm online at about 14-16Mbit with no Virgin style traffic throttling