I migrated to Plusnet FTTC last month and my experience has been similar to the other posters on this thread:
First line support via phone (or the online trouble ticket system) are not "script driven" and will listen if you demonstrate that you are technically competent, although you may need to be a bit persistent if you have a complex fault or something unexpected.
Like markd66, I didn't have a trouble-free migration. I was one of those who got accidentally put on the wrong BTOR FTTC service (40/10 instead of 80/20). This was rectified within 2 or 3 days, and whilst in an ideal world I felt it could have been handled a bit better, I was also very aware that many ISPs would have handled it a LOT worse. So overall, I was fairly satisfied with my experience of Support on this issue.
As Oldjim and RobertoS have said, the Digital Care Team make a big difference - they are active on Plusnet's own forum and (to a slightly lesser extent) on here. My observation is that they try their best to pick up issues that need chasing or escalating and get them dealt with. They are also pretty open about what's going on behind the scenes. For example, there has been an issue affecting throughput and latency during the evening peak on some gateways over the past week or two. This seems to be quite a complex technical problem and not quick or easy to resolve. They seem to me to have done a good job of handling this through the Plusnet forum: being open that there is a problem, engaging active members there in providing diagnostic feedback, providing status updates, and dealing with specific customer complaints.
And finally, as has already been said, Plusnet is reliant on BTOR for provisioning and maintenance of the access network. From my recent reading of this and Plusnet's fora, it seems that this is where a lot of the real nitty-gritty problems lie. As a past customer of Zen, I would say they worked extremely hard at developing a relationship with BTOR and actively managing it. So far, I get the impression that Plusnet does not have quite such good control of it's supplier (a pretty challenging task for all ISPs!). It's probably a lot harder for a larger ISP. I'll leave you to judge how well Sky and BT Retail do in this regard.
Like you, I looked at all the (affordable) options for unlimited FTTC before opting for Plusnet. So far, I think I made the right choice.
PSS/BT Gold > Compuserve > Dircon > Zen > Be Unlimited > PlusNet Unlimited Fibre