After a lot of indecision, I have finally switched from Zen Internet to Plusnet. I suppose this is a bit like changing from BMW to a Ford. However, my experiences thus far have been OK; other than a lack of clarity about the procedure of switching an FTTC connection (rather than installing a new one), Plusnet's Sheffield-based customer services have been fine. In fact, I'd go so far as to say that they have been just as good as Zen's.
Of course, it's the same BT Openreach FTTC connection for the last ½ mile, so my headline speeds are the same, and Plusnet's well explained traffic shaping (versus Zen's complete absence of traffic shaping) has been completely unnoticeable. The main thing though is that it's unlimited (versus a 200GB monthly cap) and 40% cheaper.
I would have saved a lot of money by now if I'd switched a year ago.