I'm about to give Zen a try on one of my lines as an experiment.
As Zen is so horrifically expensive (£79+vat) for a business grade ADSL connection, I'm sticking with Demon elsewhere unless there's some unbelievably compelling reason. I currently use the Demon Business Broadband 2000 service at £34+vat (or £40+vat for a notional 8Mbps).
Demon's customer service is diabolical, for business users, at any rate where anything concerning ADSL and BT Wholesale is concerned. But their prices and no-nonsense usage allowances for business users are as far as I'm aware unmatched.
Zen would seem to be a good choice for anyone for whom cost is of no consequence, or people who are extremely light users.
