Have to disagree completely about Zen CS.
I was one of those who ran into a seemingly perennial problem with single threaded download performance (now only on the third page on thinkbroadband's Zen forum, so perhaps they've finally sorted it for everyone?). It was monumentally difficult to get Zen to listen, and frankly some of their customer "service" responses would make TalkTalk look like their CS was staffed by the likes of Vint Cerf and Tim Berners Lee themselves. Not even raw, hard data could sway them.
Only with sufficient escalation (via thinkbroadband) to an actual network engineer was progress then made, though it still took a long time for it to be resolved.
A problem I have never, not once had on my BT line.