Hey, I moved from O2 which gave me a stonking 3-4 megs on a 59db line. [When with Orange, but via BT, I often ended up at 135-160k for weeks on end!).
O2 were great, not a moments bother, and generally speeds in the range above.
Moved to Infinity, and get 37.5 megs via ethernet (and around 20 megs on wireless g). This has been stable and at this speed since November. On the couple of occassions the speed dropped (down was only 6megs and up was still 8megs), a call to offshore immediately escalated it back to the UK and whatever it was got sorted very quickly (excellent performance this time). To be fair, the BT site did say there was engineering work going on the night before each of the issues, and I wonder if there was simply a delay in information propagating around routers etc. They also called me the next day to see if it was sorted.
The only question for you is I suspect is not the download speed, but the 10 megs upload - which I can say does help things greatly. Emails zip off in a flash, photos load up to Flickr etc in a blink of an eye - makes life so much better, believe me - I did not even think it was an issue until I switched!
And of course, there is also the 80 megs on the horizon too.
As others have indicated, most cabs only have 288 fibre connections, so in high demand areas, these can be used up quite quickly, and then you won't have a choice. My cab only has 144, so I jumped in asap as we have loads of houses here on the estate.
Hope this helps,
1999 Freeserve Dial up --> Freeserve Anytime ---> 2005 Wanadoo 512k BB ---> Jul 2009 Orange up 8 Megs(hah!) --> Nov 2009 O2 LLU -> Nov 2011 BT Infinity( getting 37.5 megs now on ethernet)
Yamaha AW16G user