I've used it before. Not for anything "production worthy", but the tool I built is seeing at least monthly use at my company, seems relatively stable (hasn't broken yet), and was way easier (for me at least) to write than the same app when I originally wrote it in Angular.

I had a bit of trouble convincing it to redraw when I though it should, and ended up throwing in way more m.redraw()'s than I probably should have, but it works, it's responsive, it's fast (if you couldn't tell that already from it's benchmarks), and it's fun (for me at least).

*Note: I haven't bothered looking into React yet, so I have no real comparison between the two. But Mithril suits my style of development, and that's good enough for me.

P.S. The templates look really nice in CoffeeScript too, for what it's worth.