I couldn't get away from Sky soon enough. Was with them on ADSL for a while, and had constant problems with the crappy router they provided. There was a bug in its firmware that meant that connected devices were never displayed properly. I either got a list of the same device connected repeatedly, or, an incomprehensible list of devices in what looked like wingdings. I complained to Sky, who acknowledged that they were aware of the problem, and that Dlink had released a firmware update several years earlier. Sky refused to release that for their customers and also refused to give me my username and password, to allow me to use a decent router.
When fibre became available Sky offered up to 40Mbps, BT offered up to 70Mbps for the same price. We get well over the 40 offered by Sky, usually hitting around 60.
The process of leaving was painful, with quite probably the rudest call centre handler in history, who was shouting and refusing to listen, intent on going through her script of bribes to keep us with them.
Once we'd left, we received regular monthly offers, inviting us back, with the discounts getting bigger by the month. Strangely the offers stopped as soon as our 12-month contract with BT had ended and we could actually take them up on any offer.
One extra bonus in leaving was that we no longer received regular calls from some warranty company, saying our Sky box was out of warranty and we needed to pay them, just in case it broke. Complaints to Sky just resulted in denials that they knew anything about the third party. A claim that seems even less believable as we stopped getting them as soon as we left Sky.
BT aren't perfect, in fact the home hubs are pretty poor, although since we got the HH6, we've not had any problems. When we did, on HH4 & 5, BT sent engineers round quickly, provided replacement hubs and also sent a powerline setup to improve the wifi in the extension, all free of charge. Whenever we were having problems, we had regular call-backs from BT staff, checking if the new hubs had solved the problems, were the powerline hubs OK? At the end of the problems, a nice compensation payout was offered, without us asking, so, couldn't fault their customer service.