I'm with BT (I can hear the sharp intakes of breath from here!), I did stray for a short while and went to NTL - who were then taken over by Virgin Media.
I had quite a few problems and in the end went back to BT, and have been with them ever since. I get what purports to be a loyal customer discount when my contract comes up for renewal, and basically I get superfast unlimited fibre for the same price as their unlimited copper cable price. Touch wood I've only had a couple of minor problems that have been fixed quickly.
I have looked at other ISPs and the only one that appealed was Plusnet (a BT subsidiary), but it always puts me off when you read the negative reviews - although I'm aware that people are more likely to leave a bad review when they've had problems, than someone who is satisfied is to leave a good review.
I tend to think 'if it ain't broke, don't fix it' and, and stick with BT.