Was with Virgin from day 1 and stayed until ADSL Max arrived and my promised 'up to 8 meg' evening downloads became slower than the dial-up modem - really, 36kbs.
Synch speed was fine, just Virgin's backbone and service that was bad.
So I went to ADSL24 (an Entanet reseller) and was very pleased until new things such as ALT (traffic throttling) became a problem, so went with their LLU service. Costs more, but my son is payiing the extra for his gaming etc. Also they have a good, UK based call centre with people who don't just read from a script.
On standard ADSL Max I got a max synch of around 4-5 meg, LLU pushed that up to around 8, and that's all my poor BT line will give me. Next door gets 12+ meg, so it's something between the green box on the corner of the street (50 feet away) and my house, but BT don't give a monkey's.