I signed up for Ethos a few weeks ago. So far, I'm very happy with it. More importantly, my first client that I migrated to the platform loves it. They were using stagnant pdfs for their brand guidelines, (gah!) so this is a game changer for them. For me, it helps me keep all my clients' brand assets in one place so I don't have to go searching through multiple files. I had a snafoo on the platform where I managed to remove myself as an editor of a guideline I created and Brian Friedman fixed it quickly and upgraded me to boot.
I like the customization options for each brand. The interface is very straight forward and has all the options you need. My only wish is that it were a prettier interface.