Been looking at these for sheet music! (The larger models.) The ability to natively run Google Play Books is a huge bonus too, very few offer that compatibility and that's where most of my books are.
I wish Movies Anywhere existed for ebooks but there's no way Amazon would go for it. :(
I own an onyx boox and I can't emphasize enough how much I don't recommend it for anyone. If I had known the company was a serial GPL violator, I would have never purchased it in the first place. My recommendation would be to wait for the refresh of the ReMarkable Tablet, as it runs GNU/Linux and there is a healthy community of people who have made a ton of applications and tools for it [0]. This will likely never occur with any Onyx products and you will be stuck with a device running an old version of Android with SELinux disabled, on a vulnerable kernel, that constantly phones home to Chinese cloud servers.
I have an olde Barnes and Noble Nook that I rooted and installed Play Books. Used that for years to read public domain books that I got from Gutenberg and uploaded to Play to get position synced between my phone and Nook. Now I mostly read Libby. I also have the Kindle app on the Nook, so I need to see if the old version of the app still works and if I can send books from Libby to it.
I wish Movies Anywhere existed for ebooks but there's no way Amazon would go for it. :(