I've been using this extension for a while and have come to really appreciate it. At first I didn't really see the purpose of it, as Safari allows one to retain the tabs and URL's from previous sessions, and having an extra tab for it open upon starting up Safari confused me. But with a few minutes of diligent checking it out, it all became clear.
Tthat extra tab or window with the Restore Session header is where one can choose to look at previous sessions (menu on LHS), and then choose to reopen any full session or portion of one that you need.
The most useful aspect, I find, is for when I accidentally close a tab, which I have kept over several sessions. If you haven't used that tab in the current session, it doesn't seem possible to access it again via Safari's history feature, BUT with Safari Retore, I can bring up the Safari Restore tab/window and go to a previous session that had said lost URL tab at the end of a session, and get it back. Very handy, particularly for single-user machines. I can see, however, how it might be a privacy issue if you share the same computer and account with someone else
Also have never found any instabilities to it that I am aware of.
And as a general comment for all extensions, seeing that it has been asked here several times, to uninstall this or any extension, just go to Safari->Preferences->Extensions (which becomes a tab in Safari) and choose "uninstall". You can also just choose to disable it.