If anything goes wrong BT are one of the worst companies I have dealt with to get it fixed. Their customer service is nothing short of appalling.
It's the industry standard. So much for the improvements we were promised that competition would bring when telecoms were privatised 25-odd years ago. BT don't get voted amongst the worst however: http://www.thisismoney.co.uk/money/bills/article-2084868/Customer-service-TalkTalk-gets-Wooden-Spoon....
When things go wrong with any provider it's usually a pain.
When I first moved to this house I retained the BT line and transferred my O2 broadband. There was a phone line issue that caused noise on the line with the router connected (no not a filter problem it's called an HRDIS fault and is caused by a bad joint). After 5 BT callouts it still wasn't fixed and I gave up on BT and ADSL and moved to cable.
Cable was good until VM started overselling their capacity by offering 100Mbps unlimited. Somebody on my little network segment must have taken it and given it some real welly because my cable performance nosedived and stayed that way. VM promised to fix it but the fix date was months away and they seemed to think that was fine. I didn't so I downgraded to 10Mbps as that was all they could deliver and resolved to give BT another chance when Infinity arrived. Well that will be in a couple of weeks time. Hopefully they'll either provide a decent line this time or the engineer doing the install will fix it if it's the same one and the duff joint is still present - hopefully in the intervening two years it will have progressed to not working at all so they'll have to fix it.
Fortunately for most people their phone and broadband is reliable and trouble free. If it isn't the ride will be a rough one no matter what technology and companies are involved.