I moved from Aquiss FTTC Business to BT Infinity in February and the move was smooth except for quite a bit of confusion at BT on whether I would need an engineer visit or not.
Initially I was told I would need an engineer visit even though they didn't seem to know why but I said no problem with that as I needed my fibre modem changing over anyway as it was one of the originals that supposedly had an overheating problem.
Change over day came and my connection was down first thing in the morning so I stuck the new BT login details in my router and got connected no problem.
Engineer never turned up and then I had a bit of a mare explaining to support that I was connected OK but still needed my modem changing. Several phone calls later I did manage to finally get someone to come out and change it.
Apart from the confusion during the changeover process I have been very pleased with the swap to BT. Aquiss have great support and generous downloads during off peak hours but at times during peak hours or very busy periods of net usage (Andy Murray playing tennis in Australia for example) their network would get very very slow. On 40Mb fibre I had a few occasions where i couldn't even get 1Mb down. Price wise its obviously also very tough to compete with BT which was one my main drivers for moving.
If support is a big thing for you though, I don't think you would find any better than Aquiss, Martin is always quick and spot on with technical help.