I moved away from Zen
...but to be totally honest, they have been the best supporting ISP I've ever dealt with.
I'm currently on O2's LLU ADSL (on an old package with no limits, a 1-month rolling contract, etc).
If Zen did the same thing for even double the price, I'd move back: that's how good they are.
You get what you pay for with them, which is: a solid service.
Even when things go wrong they are good to deal with, in my experience. They don't dumb down their service either: if you want to do something a bit interesting like running the DD-WRT firmware on your own router, they don't mind, and will provide you with things like ADSL encapsulation settings and stuff without complaint or confusion. Try doing that with any* of the volume ISPs and see where that gets you!
* (Except BE*, who are also excellent)