I originally went for Zen specifically for the 1 month contract. I was waiting for Talk Talk to activate in my area and was going to swap to them.
However I've been so impressed with Zen and have read all the horror stories with Talk Talk so decided to stay with them.
Been with them since May last year and had no problems at all.
(Well two problems but one was BT ballsing up at the exchange and the other was my router throwing an unexpected fit, so no fault of Zen's).
I get regular download speeds in the range of 750 - 900k/sec