I actually belong to a consortium member who is not currently using Contact Permissions.
I've converted several organizations from using other various approaches (attributes,email/mail/phone indicators, etc.) to using Contact Permissions. It's a bit confusing to some end users up front, but ultimately permissions work much better and provide more consistent results from what I've seen.
I'm waiting for v16 and will add the Plug In. I have it mapped out and I envision it as a great improvement.
We're beta testing v16 so rather than having to map it out multiple times I went ahead and put it in our live system. I agree that it can be confusing at first for end users but am overall happy with the change.
My concern right now is that I'm looking at rather a lot of Permissions: basically we have a matrix of Email/Mail/Phone with things like Any Contact/Marketing/Soliciting/Trade and a bunch of other stuff. Even crossing out the stuff we don't ever do (such as telemarketing) we wind up with 11+ Permissions. Our goal is that if a customer responds to remove themselves from a solicitation email that they not be also removing themselves from marketing emails, or if they ask to not receive marketing emails that not preclude mail. And this is ignoring various channels that we currently control via Interests or Contact Point Purposes.
The conversation I had with Tess support is a Global Permission for optin/out and interests for communication channels. Of course for us we'd need a Permission for each external org that we are partnering with (and capturing opt-ins for). The big thing for me re permissions hinges on the plug in functionality and the ability to raise opt-in requests for specific products in specific sections of the pathway. It helps to map that out.