It has lots of unexpected options that are nevertheless intuitive and easy to use. Important entries can be protected from automatic deletion. Even the number of times an entry was used can be edited.
It's amazing to see, with the help of this add-on, what data FF keeps storing, even those data that I thought I had deleted manually from the search fields (via "highlight + del button"). Also found some entries (account numbers, etc.) that I really didn't want to see stored.
IMO, this add-on is an absolute must for privacy conscious users. It should also get a prize for combining utmost efficiency with simplicity of use.