I've been with o2 Broadband since the start of the year, so have a few months experience under my belt to make a review.
I had quite a few reservations after leaving the perfection of Zen for o2 but needn't have worried. At Zen I had reliability, good speeds and great CS but paid a premium price for 20GB bandwidth a month.
The differences with o2 are all positive in so much as that i'm now paying less then a third of the price and have an unlimited bandwidth connection. I think they call that a no brainer.