I really like that this exists, but I found the menus and UI to be rather non-intuitive. My main use case was to sync reading progress between my Kobo and my iPhone. I was able to do this using KOReader on the Kobo & the Readest app[1] on iPhone (also open source).
Ultimately I decided to get rid of KOReader because it felt a bit laggy. The gestures (eg: swiping up on the left side to increase frontlight brightness) didn't work well either.
I eneded up writing my own software to sync progress from Kobo to a KOReader Sync server[2] which my Readest app also connects to (love that unlike the Kindle, the Kobo is quite an open system). You can even set it up so you can SSH into it[3].
[1] https://github.com/readest/readest [2] https://github.com/koreader/koreader-sync-server [3] https://www.mobileread.com/forums/showthread.php?t=254214
I really appreciate that KOReader is an option. On my older Kindle (first-gen Paperwhite), the default reading experience had declined until page turns take several seconds. Firmware update and factory reset wouldn't fix it. Using the default reader just wasn't a viable scenario.
I jailbroke it, launched KOReader, and it's like I have a new e-reader. The amount of customizability gets kind of in the way, but once it's set up I don't really think about it from one month to the next. I also didn't feel the need to upgrade my original install either.
I tried to set up KOReader multiple times and always got stuck figuring out where all the settings are and how exactly they work. It seems very complete, but I'm just too dumb to actually use it.
It's good to have options.
Most software has compromises.
KOReader seems to have functionality in one place in one reader app that other e-reader apps don't.
https://github.com/baskerville/plato might interest you. I've yet to try it, personally.
I also found all existing e-reader software to be lacking, and all I really wanted was a basic reader. I ended up writing my own with Rust and MyPDF (the latter doing all the heavy loading).
The hardware is great, but the software is pretty lacking. Apparently a lot of the tools out there are from folks in similar situation: had the hardware, but the software to drive it was lacking.