I joined on 20th April. The literature I got said it would take up to 1 hour to get the fastest speed and in rare cases 24 hours.
I was on 2Mb before and was getting around the 1.9 download mark. With the "up to 8Mb" I am getting the same speed, although BT said I would get about 6.5Mb when I checked on their website.
I phoned and emailed BT and was told to "wait 5 days" and get back to them if things had not improved. A help line guy even said they allow up to 10 days for the speed to get up to its highest. I am still running around the 1.5 to 1.9 mark so may phone them tomorrow.