I have been running a 7800N on the Zen Unlimited Fibre 2 since I joined them last year. No problems, although I am not getting quite as good a download now (initially 74, now 72) but upload has gone from 17.6 to 18.3
All in all very happy with my speeds.
Latest TBB speedtest
BT (poor) -> Zen (excellent) -> O2 (started well, went downhill -> IDNet (No complaints - but 100GB cap) -> Zen (unlimited)