I'm one of the aforementioned creators of Trailmix. I am indeed a thoughtbotter, but Chris is actually a GitHubber.
So far, our $3.99/month price seems to be working out well. We get the occasional comment that the price is too high, but people are still signing up at a steady clip (~5/day).
I'm glad to hear that you (eventually?) plan on charging for your site. While it doesn't cost much to host something like this, the opportunity cost of time spent on support and maintenance is real. I'd be uncomfortable using a free, freemium, or "pay what you want" model given that. It's probably fine for your first year, but as your free user-base grows and your passion fades, I imagine it's tempting to shut it down. After all, the OhLife guys used a freemium model too.
Anyway, best of luck to all of us!
Our site: https://www.trailmix.life
The code: https://github.com/codecation/trailmix