I can only speak from my experience of the fibre upgrade process...
1. Some users are reporting it takes time for Plus.net to recognise a new connection once the router is plugged in. This means Openreach do not wait when they do the install. Does this mean I can't use my own router as Plus.net routers does something special?
In my case the Openreach contractor didn't want to hang around while the PlusNet router got itself set up. I was very unimpressed by his whole attitude. You might be lucky to get a more conscientious engineer (maybe even a genuine BTOR one).
From what I have been told on these forums I understand that the 15 minute period is just the time it takes for the new router to be recognised on the network and then configured with its broadband username and password using the TR069 remote management service.
I can see no reason at all why you cannot use your own router as long as it supports PPPoE and you set it up with the right parameters. Equally, I understand there is no problem with using your own VDSL modem (usually built into the router) to replace both the BTOR modem and the PlusNet router. If you have a line problem you'd need to put the BTOR modem back so BT can run their diagnostics, but I'm thinking about getting a Fritz!box or maybe a Draytek 2850 to replace my BTOR modem and PlusNet router...
2. To get the best deal you need to sign up with line rental what disadvantages does this have moving away from BT?
Haven't figured out any yet apart from having to pay an extra £1 per month for CLI. I'm not sure if there is going to be a cost to move my line back from PlusNet to BT in the future, but if there is I think BT normally waive it if you sign up for a 12 month contract. Others may have more info on this - I've only just joined PlusNet.
3. A self install option would be great - difficult to arrange being in during the day. I have an original BT ADSL master filter and the modem would connect to this - straightforward setup. Are there any realistic chances of self install becoming available soon?
This site reported that there are some trials of self-install going on, but I'd be surprised if this gets rolled out in the near term. The BTOR engineer has to visit the cabinet, a few hundred metres from your front door, so it's not too much extra effort for him/her to come to the house, plug in the modem and check it works. I guess it depends how the trials go.
ISP: PlusNet Unlimited Fibre