I use a Boox Note Air (original) for some stuff. I find it useful for being able to archive notes and access them anywhere - write on the Boox, read on the PC or phone, and vice versa. My previous paper notebook I scanned and then synced with the Boox, so there is old reference stuff too. IMO, the ability to back up the data is a big plus.
But... I miss colour. Sometimes I used red and blue pens to highlight stuff, and you can do that on the Boox but it shows as either black still or some laggy halftone (but is the right colour when seen on a colour device). I might take a screenshot of a scope, print that and glue it into a notebook and it will look good. I can do the same to the Boox but it's greyscale and the effort seems almost more than the printing and gluing! Perhaps it's the lack of colour at the end that alters the work to payoff balance.
However, I can send screenshot to the Boox and then draw and write on it. I can do that with the printout in a notebook, but I only have one chance to get it right. On balance I think the Boox outweighs the notebook (at least, I haven't gone back). I imagined it would also prevent the scribbling on lots of scraps of paper, but that hasn't happened yet.
Mostly I keep WiFi off, so there is no chat between the Boox and its masters. I turn it on for syncing, obvs., but I use Nextcloud as the sync server and a common Android client for the front end. There is nothing dependent on the cloud, and while there may be some background chatting under the radar to foreign parts it's a very small window for that to occur. In fact, the only reason I haven't thought about just blocking it at the firewall is for the firmware updates check. Of course, this is the version 1 and the new version may be different.
I have sometimes wondered about using an iPad instead (the videos I've seen of one in action show it is very slick and colourful) but I prefer epaper for this kind of thing. A benefit of the Note Air for me (and also a downside) is the glass front - I am not a fan of the plasticy alternatives, and the glass gives you a flat facade (the plastic ones have a dip in which the screen resides). Applying a screen protector is a lot simpler give that - it doesn't have to be the size of the active screen, just needs to be symmetrical (for looks).
Why a screen protector on a glass screen? I use one solely for the feel. The glass is a bit slippery so feels like writing on glass. The correct protector makes it feel like writing on paper.