I only have a PAYG mobile but moved my land line service and BB to O2 last year. Twice I've had line problems - a noisy line interfering with speech and dropping the BB connection - which isn't really their fault, it is a BT line after all. By the nature of these things, intermittent faults, it took a little while to sort. And eventually they -and Openreach - did by replacing the line to the exchange. Throughout the impression given was that you weren't being a nuisance and they really did want to solve the problem.
As you say, Covergirl, a completely different experience from BT.