At the moment I use MainWP only for myself, to make it easier to manage my own five websites. But I like it a lot. Even with just these few websites of my own and no clients to manage, it does make things more streamlined.
For my needs, Pro is an unnecessary expense — but I got a discount code and after some procrastination I treated myself anyway. Perhaps I’ll find some practical ways to justify it financially in future. 
Nice product, keep up the good work team.
If there was one thing I could wish for though: Some kind of automated update system that detects when an update breaks the site, rolls it back to a known working state, and alerts me. As a non-coder, updates seem fraught with potential to go wrong, and while MainWP can automate updates I still live in fear that my sites will break.