I installed Debian Jessie alongside PureOS on my Librem 13, because I’m very picky about building up from a base-install among other things.
I did the install with only the “free” repo (and later added contrib to install torbrowser-launcher).
I used Jessie (stable) with the latest backported kernel 4.2.6 in jessie-backports.
Here are a few things that I noticed / am tweaking:
The kernel warns about possible missing firmware for the Realtek ethernet adapter - you should ignore this, ethernet works perfectly without needing any non-free firmware.
Touchpad / trackpad - I’m not having much luck with multitouch in my Debian install, but it works in PureOS (which looks to be based off Stretch [testing]). I assume that I’m missing a package for this, or need to install a newer version of one and haven’t done much research. I have xserver-xorg-xinput-multitouch. I’m also using enlightenment 17, which is rather finicky.