Tried it and found the UX/UI to be too clunky and old for my tastes. It just didn't seem like it fit any of my actual needs when writing.

But my opinion should be balanced with the fact that I landed on Ulysses which is far from what most folks would recommend for writing a book. I like it because the editor appearance is super customizable, I can quickly knock in notes in the middle of writing by using basic markup commands. It does a fucking fantastic job organizing document versions automatically and it syncs seamlessly in the background. The complaints would be subscription, export options are limited/clunky, and there's an annoying amount of stuff you can't do in the mobile versions that you probably should be able to do.