I can only echo the BT experience. Switched from Sky due to the faster broadband, also the cashback and sainsburys voucher. Cancelled with Sky was supposed to switch a month later, sky turned everything off as planned, got to the end of the day and nothing from BT, called them and they said there's a problem switching the phone number we'll have to place the order again and it will be a week, super a week without broadband, week went and the phone was installed, but only the phone, called again (they never once contacted me to say they'd fucked up) and they said that broadband and TV needed to be put through again and would be another 10 days, oh and the phone had added a £50 activation charge which wasn't part of the order, the cashback wouldn't be eligible and doubtful I'd see the voucher, was promised call back after call back to confirm and sort the order.
In the end I got fed up, emailed the director and said I want it all cancelling and I'm going back to sky, slow broadband is better than nothing, got a good deal going back to sky. I've now been assured I'll be credited everything back and have a UK contact to resolve anything should it go wrong and this isn't their usual service (it clearly is). Nobody in the Indian call centre does what they are supposed to and once you're off the phone you no longer exist.
So for the ramble, but in short avoid at all costs, absolute nightmare. Given the choice I'd be with Virgin, but I can't get it here, good broadband and never had a problem, nor sky to be fair, just the broadband is 38mb, where as BT was 76, although they presumably use the same wires, so not sure why that is?