I've been with them for 18 months and the connection has been rock solid with no down time. Speeds have been as predicted and the one problem I had was quickly sorted by the OCE's on the forum. I am a happy TalkTalk customer and so are three other members of my family. You can only speak as you find, and my experience of this budget ISP is good.
I recently moved (not much of a move really!) from Onetel to TalkTalk; after a week or so I found myself without broadband. The TalkTalk customer service people I talked to were very patient, helpful & knowledgeable. As all the standard checks (swapping filters, connecting direct to BT box,etc) didn't work & the line checked out OK they arranged an engineer call-back for the following day. As it happens everything went back to normal after a few hours, so I rang to cancel the call-back - they said they'd leave it in the schedule in case I had further problems.
Others' experience may differ; as far as I'm concerned TalkTalk's service is excellent. Don't believe everything you read in the Daily Mail! The problem with such sensational "journalism" is that they just use bare numbers without looking at the broader picture. So often they say that Company X is the "worst" in its particular field because it has the biggest number of complaints without relating number of complaints to number of customers - make that comparison & you'd probably find that their "worst" is in fact one of the best!
As I think we've said before, consumer judgement about any service is ultimately best expressed as a percentage risk. In that sense you take a bigger risk of bad service/support with TalkTalk than the other big ISPs but ultimately most people still get a good service.