I got an Xteink X4 a few weeks ago and love it. I've tried out a few different firmwares but haven't settled on one yet. I love the screen and how simple the interface is. It's kind of a pain in the butt getting my Kindle books on there, but worth the effort. It's also encouraging me to buy books outside of the Amazon ecosystem. I've been getting more stuff from Humble lately.
Is there a non-annoying way to get Libby books onto it? It's a step or two with a Kindle, but it would be cool to have them on a more portable device
I also got one a few weeks ago! I love how pocketable it is.
I tried different firmwares too and ended up settling on CrossPoint. So far it's been the most stable and the UI feels more intuitive than the others (for me at least)
How do you get your kindle books on it?
My favorite at this point is Witch reader (https://github.com/jpirnay/witchhunt-reader). All the features of Cross-point, with better book rendering, and more features. I've tried a lot of the various alternatives, and keep coming back.
If you want all your book series displayed together, AALU (https://github.com/dawsonfi/aalu) might be worth a look.
And Crosspoint OS itself is still really nice.