* The writing experience is top notch. It feels like pen and paper. I recently had the chance to try Onyx Boox Max 2 (very briefly), and it's nowhere close to the RM as far as the tactile feeling goes. This is the main selling point of the RM, and it's very good.
* It's an open-source Linux-based device but not entirely. The actual UI application isn't open source. But as others have said here, it has pretty good hacking potential, and it's definitely more open than most commercial devices. The company's support of open-source was another reason for me to buy it.
* The on-tablet software started out really bad but has improved. The first version had a bug that dramatically reduced battery life. It took a few more updates to get essential features like inserting a blank page in the middle of a notebook.
* The off-tablet software is bad. There's a file manager webapp with remarkably poor UX, the cloud sync API isn't particularly convenient for automation, and so on.
* As a reader, it's mediocre at best. PDFs formatted for large screens look good, but rendering is slow, navigation is poor, etc. I would absolutely not replace my Kindle with the RM.
To summarize, I think the RM is a very good device if you treat it as a digital paper replacement. That's what it is for me, and I'm happy. But if you want a good e-book reader, or any advanced software at all, this isn't the right device.