I just switched to Vodafone after using Zen for so many years(16+). Reason for transfer was being on a business tariff and realising £50 a month for BB and phone was a bit high. Contacted them in the past about it and first time was within a contract period, due to a house move and they said I'd have to pay a cancellation fee to swap(weird considering my years with them), and second time they called once, never left a message and didn't bother calling again or emailing. My connection has been a bit dodgy lately although I don't blame zen for that (BT, yes), so decided to transfer to VF to try another service for a while if I'm not going to see my old transfer speed again - I used to see 80Mbps at one time. Had to call them 2-3 times a year recently to get someone out from BT to check things over, each time finding a fault, usually with their own green cabinets that they seem to be always going into, daily.
Anyway, VF still balancing the line at the moment (day 2 only) but it's dropped to 62 from the 69 I was recently getting with Zen so hopefully that improves as they did say the speed should not drop. I've also had an issue with DNS which has been a right PITA. FIne for normal use but couldn't get to some servers behind a company VPN for some reason. Ended up having to change the network adapter settings to use other DNS servers(google) for both IPv4 and IPv6.
Still, free for 6 months and nearly half the price of ZEN per month after the free period, and with an additional £2 off for being a mobile customer, I'll see how it goes for the next 2 years.
Zen support were indeed very good at helping get issues resolved, ie, diagnostics and then engaging with BT/Openworld to investigate and fix issues.