I'd rate the experience of using this addon as very bad.

First of all, I did not experience any losses of stored sessions, saving them, as well timed autosave, which is one of the most important features, do work.

However, the process of restoring a session is beyond horrible - every single tab is getting loaded into memory.

When trying to restore a session with this addon, Firefox ate around 12GB RAM, basically all available RAM I had (it would have probably ate all 32GB if I had them), and made PC completely unusable, it lagged to hell. I waited for around 20 minutes, but nothing improved, so I just gave up, and closed the browser (it crashed on close too).
After that I was able to use native browser session load, without problems, with all tabs not loaded into memory, like it should be.

I tried using "Load on Select" before, but it has problems with some of the sites - they do not load with this addon.