We have O2 and it has been fab so far...but e did have AOL so they weren't hard to beat . I think they are officially ranked No1 in customer service ATM (according to their advertising). probably true. They are powered by a company called SimplyBe which has always ranked good.
My mum had Wandoo (which became orange) and it was crap.
here is the mumsnet equivalent of geeky broadband stuff you can compare ratings from Thinkbroadbanders here.
They also have a forum which is divided into customers of each ISP at the bottom so you can check out each provider. They are really friendly...just a little lesss chatty than us.