I would say this from observation.
Much cheaper buy out, the figures RobertoS posted are less than half of the BT buy out cost for contract.
Static ip availability on consumer accounts.
UK based support and forum based support.
Overall cheaper price.
If a routing/network problem, it seems one of their head guys is active on forums, whilst at BT it is nigh on impossible to report to the right people due to it been a big organisation where those people are kept away from customers.
Advantages of BT.
Higher network capacity (although this is unproven.)
Easier to get expidited installs/engineers.
No prioritisation on traffic.
Unlimited usage 40/10 service, this is attractive to those with low syncs but still want unlimited use. Or eg. it could be used to keep a dodgy line on fast path whilst keeping unlimited use so eg. if line syncs at 60/13 but has issues with stability, then change to 40/10 service and line has excess snrm making it stable.
BT Infinity 2 Since Dec 2012 - Estimate 65.9/20 - Attainable peak 110/36 - Current Sync 46/20, Current Attainable 74/29