I've got a ReMarkable (got it for free from a friend who never used it) and I think it could be great depending on your specific use-cases.

It's a pretty great tablet for taking notes on, using the pen to write feels like writing on paper, the writing is responsive, and now they have an optional keyboard for typing things up on it for situations where that works best. The biggest downsides on the note-taking front is the difficulty in adding new templates. It comes with 15 or so templates, and you can add more, but you need to be somewhat tech savy (basically if you can work SSH and know how to move files around on the CLI, you'll be fine). Though the added templates won't be able to display previews properly, but that's not the end of the world IMO.

As an E-Reader it's functional, but a bit subpar. The screen size is awesome for reading, I use a somewhat small font, and so I can fit a good chunk of a chapter on a single page, which is nice. The built-in E-Reader is basically useless, but you can download KoReader on it (again, some proficiency in using the CLI is required here), which is excellent. The biggest issues I have with it's ability as an E-Reader are the lack of a backlight (so no reading in bed without a reading lamp) and - somewhat ironically - the size (which is both a blessing while you're using it, and a curse while you're travelling with it).

It's somewhat (but not really) FOSS - they use a proprietary flavor of linux under the hood, and you can access the base operating system over SSH. If you're a linux guy/gal, it can be pretty cool, since you can install all sorts of stuff on it and setup recurring jobs via systemd (for instance, I wrote a short systemd service which swaps out the screen saver image every 5 minutes while it's in use). This also means you can potentially brick your remarkable though, and it's not easy to unbrick (though it's possible most of the time using a USB-C breakout board).

Overall, i honestly don't use it a whole lot, but if i was the sort of person who liked making handwritten notes, I'd probably use it a whole lot more. But I generally find it easier to just use my kindle for on-the-go reading, and my phone/desktop for note-taking