I am on BT's G Fast trial for business users (£1 a month for six months) and thought I would share my initial thoughts.
I was previously on Infinity 2 and given I can see the cabinet from my window got speeds of 78 down and 20 up.
After booking the upgrade I was sent a BT Business Hub 5 (which I already had anyway).
On installation day the engineer went to the cabinet to switch my line to the new service and then fitted a new faceplate to my master socket, He then doubted the Hub would connect to the service.
After some investigation it seems the trial on business and residential lines is being conducted differently. Residential get a combined hub and modem. Business use the Hub 5 plus a Huawei MT992 modem.
His own tests gave speeds of 327 down and 45 up. (There is 63 metres of copper between me and the cabinet). That was using his test gear. His BT supplied phone app cannot do speeds over 100Mbps!
On connecting up my network we got speeds of 92/23 on th BT Wholesale checker. Engineer said that was nothing to worry about as the tester also wouldn't go above 100.
Turns out this is wrong. Problem was two-fold. Firstly the network card in my PC was locked down to 100 for some reason - so it was never going to report a speed above that. Secondly the cable connecting it to the hub wasn't in its first flush of youth. Experimenting with various different cables and routings has now reported speeds of 309/31. I also get consistently better speeds if I set the network card speed to 1Gbps rather than auto negotiate.
I have also tried the newly supplied Hub 5 and my original one and get similar speeds so it seems the Hub has not been tweaked for Ultrafast.
My line is still in the 10 day settling in period and I have noticed at times that the speed drops to as low(!) as 130.
I also have a Cat6 lead on order to see what effect that has.
Using WiFi on my phone and the 5GHz connection I get speeds typically of 180/31.
So overall I am pleased with the service I am getting. My primary reason for going for it (apart from the initial price) is the higher upload speed. I send a lot of large files to the cloud so a boost in that speed is welcome. I am hopeful some more tinkering and the new cable will increase that further.
Download speeds above 100 won't really affect most day to day users at the moment, there are too many slower points elsewhere in the chain to take advantage of it. It does work well with downloading my files from OneDrive however and they are noticeably faster.
Hopefully as the engineers do more installs they will avoid some of the pitfalls and assumptions I experienced - in the meantime this post might help you if you are joining the trial.