Demon to Vodafone
I was with demon from 1992 (ish) until last week and been through all the upgrades: modem to ADSL to ADSL2 and now to Vodafone. Never had any complaints.
I took up the Vodafone offer of FTTC VDSL. It's only been a couple of days and seems to be going well and connecting at 40Mb, whereas ADSL2 was only capable of 14Mb. I also moved POTS to Vodafone too. Combined it is 50% cheaper.
Demon was a great server provider. Remember the days when your ISP also included some free personal webspace? Going to keep demon.co.uk e-mail address.