Like all these big suppliers, when it works it's great but when there's a problem then their customer service is just diabolical. I cancelled my sky subscription out of protest of their awful customer service when I had broadband issues. I'd never use them again TBH. BT are no better, in fact they're so much worse.
I went for Zen internet a few years ago and paid £20 per month for 4gb download and they've been exceptional. I upped my service to fibre broadband a few months ago which is £35 per month. I've had a few line issues when being part of BT wholesale and Zen stepped up and got on with it. They were a breath of fresh air and I've always felt I was talking to real people and not people running through a script.
Personally, I doubt I'll go back to any of the big companies because although they give a great sales pitch,they prove themselves when problems arise. I spoke to a BT sales rep just last week and we worked out, together whilst on the phone, that they could in fact save me more than £200 per year in my phone and broadband service. She wasn't really surprised when I told her I'd pay 5 times that amount per year for good customer service.