Have used them for around 4 years and have always found their service good. I have not had many problems, but when I have had the odd problem it's been sorted quickly.
They did go through a bad patch a while ago regarding email, but it never hit me.
I have recommended them to others, and they have also had good service.
Going on what I've heard, ZEN are supposed to be about the best. But as I've had no real problems so have never considered changing from PN.
P.S. if you do go with PN use my Id as a referrer RCCLTD. Cheers.