I'm in much the same position as Matt. Broadband with Plusnet since 2002. Things went rather bad before the BT takeover c2007 (anyone remember the email loss fiasco?) and I've had some billing issues but overall I can't think of another low cost ISP I'd rather be with. Having about £12/month referral bonuses helps too

Of course I could pay more and get top notch support from Zen or AAISP but my next move could well be to Community Fibre when they pass my house.
My big gripe is with BT who told Plusnet to stop offering business broadband this year. I'm the treasurer of a charity and of course we went to Plusnet.. They made an error wit the final bill but one phonecall to a friendly Yorkshire person sorted that. I'm still fighting BT to get the billing correct.