Ive always had good customer experience from o2 as well. I can see a better infrastructure with BT though, one of the reasons I changed years ago to them was because their signal was generally better than everyone elses. Where I use it it is generally good with solid 3g coverage except Plymouth city centre where during the day it struggles to get more than 0.01mb download or upload. Its not good enough, but 4g will improve things when it eventually comes in.
There will always be a market for cable through business use but I can see how landlines for home use will diminish. BT customer service has always left a lot to be desired from what I have seen though.