Can't comment on Talk Talk, but personally I'll never go back to BT ever, ever again. I waited three months for a BT engineer to come and install phone and Internet in my current house. Had six missed appointments. Thought I was tied into them as I had moved house, and had six months left to run.
Eventually got fed up, rang Sky, they gave me an appointment for 1 week later which was kept, and I was up and running.
As some may know, it was a BT engineer that was sent out, contacted by Sky! He said it's common - the other companies that contact them get money back if BT fail to keep their appointments, so it's the BT customers that lose out.