We used ProseMirror in the past few months to build a rich text collaborative editor. We were able to get started and get a collaborative editor up and running in ~1 hour. This was a big deal for us, we'd tried to implement a good collab editing experience multiple times and each time we felt like the editor was the sticking point in creating a good experience. ProseMirror treats collab editing as a first class use case and makes it really easy.

We then spent 2-3 weeks getting all of the formatting, tooltips and key mappings to handle some basic formatting (bold, italics, underline, bullet lists, numbers). Eventually, once there are more examples on the web of more complex formatting for the community I think it will become much easier to use.

It's the first editor I've used that felt like it made sense at a conceptual level but implementing "simple features" can feel very hard at times. I don't think that's a knock on the library though. There's a lot to learn and understand and it just took us a lot of time to get it all.