I use Roof and love it!
With new tenants I just tell them this is how rent is paid and how maintenance requests are conveyed. They love and appreciate it. Even if they are paying the $2. Larger groups really like the roommate side of the app. With current tenants that I am trying to switch over I ask them politely to switch, I pick up the $2 cost and they seem to like the convenience of not having to write a check and getting the rent reminders. It typically takes a few reminders but they end up doing it. I can see it taking a full turnover cycle to get everyone on board if you don't nag them a bit. Millennials can be "busy" so snagging their attention can be a challenge. Ironically one of the reasons I love the app (once my tenants start using it) is because it moves communication out of email and straight to 'notifications' which millennials are more likely to respond to then emails or phone calls.
I definitely use the maintenance feature. Roof ends up being a automatically curated todo/reminders list app for me on my phone and a way to keep an eye on the broader details of my portfolio on the web. I can open the app and see who needs attention, or that everything is running smooth. So nice.