Must admit I like Zen, price point per service provided is spot on.
If you can get on their own LLU then I wouldn't hesitate to be honest as it is one of the best networks.
Customer services are quite good, I wouldn't say 'perfect' but then again who is? Very polite and knowledgeable even when dealing with major problems [not their fault, BT's]
I wish I could get on their LLU unfortunately I'm stuck on the BT network for the backhaul.
Go for it.
ZeN Line 1 BQM
ZeN Line 2 BQM
BT Backhaul sucks