It's almost impossible to get a subjective answer to this question, you've had two people saying BT are good, but for every one person that says any ISP is good you can find somebody else that will say the same ISP is rubbish and stay away from them at all costs.
To be honest with you very few problems with any of the main, larger, ISPs are actually the ISPs fault, most of them are either the user not setting it up properly or caused by a phone line problem, which you'd get regardless of which ISP you go with.
You can read reviews of lots of different ISPs here...
http://www.broadband-...uk/broadband-reviews/
but keep in mind people will always complain if they have a problem, very few people bother to post reviews if everything works fine.