BT here, although for the Broadband only.
IME, the service is very good, but the customer service is dreadful. (This is why we no longer have television from BT). Our TV box developed a fault receiving signal, so I tested it on the other (known to be working) aerial in the house, as well as testing a TV on the box' aerial. That done, I explained the situation, and my testing, to BT, and asked for a replacement box (we'd been customers then, I think, for five years or so).
BT wouldn't change the box without an engineer visit, so not only would we have had to wait in for them, but they'd have charged £150 for the visit if they decided that the box wasn't faulty. (I was confident in my diagnosis, but by this point, less confident in BT and their staff) so I told them to cancel the subscription and forget it.
The broadband is reliable and very fast, but the call centre is difficult to reach, limited to dealing with you in a scripted way that gives the agents very little discretion in their dealings with you, and things get pricey if they decide that the issue isn't with their equipment.