I am using BT and I can't fault them. I pay for 8MB Max and get around 6.5/7MB average. I have had occasion to ring their helpline twice, and they have solved the problem quickly. I have only ever used Tiscali before (disaster) so I can't compare BT with any others.
I have been with Sky BB for over two years with no probs. I have never needed to contact them. My phone service is still with BT! It used to cost me £5pp which was excellent value. It now costs £10pp which is still good value.
I've been with o2 for a year (moved from BT) and as I had an o2 mobile for years, they gave me the first year free. It's now increased to a massive £5!
Customer service is free, excellent and always available. The people have knowledge of the product and don't merely read from scripts which quite often have no relevance to the questions you're asking.
My exchange is not yet unbundled so for performance BT comes out best.
However, BT costs £24+ as opposed to O2's £17 (Sky's only available package also costs £17, plus £30 connection fee and a monthly download limit), so do I want to pay an extra £7 pm, £84 pa, for a promise of better performance which may or may not materialise?