I've been on Zen for nearly five years, ever since my exchange got broadband. I've never had the slightest trouble with the service, the billing or the upgrades from 512 to 2mb and then Max (heady days indeed). However, I've got the same issue as many - not enough bandwidth for a predictable cost. With two teenage kids in the house, I've been around the 60gig mark for the past six months, which is effectively costing me an additional £10 per month, taking the cost of Pro up to £44.99
Yet for £39.99, I can get Home 90 with Enta, with a 90 gig allowance (peak) and 330 gig off peak (which includes weekends). I'd have been willing to pay a bit more to stick with Zen, but there seems to be little upside. So regrettably, next Wednesday, BT willing, I'm off.
It may still be a bad move, but given my (kiss of death here) stable line, high speeds and lack of problems, simple economics says it has to be worth a try.