KOReader is one of these projects where idea is fantastic, it's impressive that it works, and the result is sufficiently quirky that I couldn't get used to it. UI is cumbersome. On kobo, it feels like it's draining battery much faster than the "regular" kobo OS. When you sync progress to KOSync with WiFi you keep getting these "Connecting to wifi" popups. And on mobile, having to use the internal keyboard feels like using the keyboard on a reader, which I do not want to.

I've never used it on Kobo (as I don't have one), but I felt that for the Kindle KOReader it was a significantly better experience than the stock experience. It's easy to sync stuff with Calibre, you can choose any TTF font, and there are more display options for reading. At least on the Kindle Paperwhite (purchased in 2015), it does not appear to significantly affect battery life.

Maybe I'm just too entrenched in the engineering space that I'm just used to "developer UI", but my wife is decidedly not a software person and she seems to like KOReader just fine.