Yes I have but not for Internet and yes I had plenty of trouble with them over 'phones as well!
My point was that BT own the infrastructure which other companies rent. If you choose an alternative supplier to BT then BT will tell you their responsibility ends at your front gate and it is up to your ISP to sort anything else out. Where it helps is if BT is your ISP they have the total responsibility and can't palm it off onto someone else. To get things done I have always found that it pays to find the name and business address of the CEO and then write them a letter sent via registered post/recorded delivery. By starting at the top and working down you may find someone that knows what they are talking about, start at the bottom and they will cover for their own lack of knowledge and inability to solve problems. Worked for me, (eventually), in UK and first time in Singapore and here in Oz.