Got this bad boy some months ago and I'm very happy with it. Its software is open source but unfortunately it's in a Chinese sort of github and I don't understand much.
Changing the font size is a weird experience since you need to use a Chinese-only program from said git repo, compile the font and its size on your computer, and then transfer the generated binary to the eReader. You can keep said binaries in your eReader later to change fonts on the move.
The formats (my few months old firmware version) supports well are epub and txt, but unfortunately things like italics or bold don't display on epub files, so I've found myself taking the epub books and converting them to txt with markdown notation for bold and italics using Calibre.
Other than those inconveniences, it's a tiny, lightweight, well-built piece of hardware, the screen resolution is really good, and it's truly the most portable or pocketable good quality eReader I've seen.