I've used it for a long time and it's been accurate until now.
I get results of 26Mb down and 3Mb up and with BT speed test & Speedtest.net 37.45Mb down & 8.51Mb up.
Quite a few people see this (including me)- the tbb test (usually) only reports about 60-70% of the speed that everything else reports on an FTTC connection. Including file downloads from non-tbb sites.
AFAIK, nobody has yet come up with a satisfactory explanation