Rated 4 out of 5 stars
Works pretty well, haven't had any issues yet and removed a ton of duplicates (especially in one of my RSS feeds). The only thing I've found thus far that I don't like is that the it's a little bit of a pain to setup. Perhaps adding a button with a dropdown menu that provides options. Included in that could be an option to scan all folders (since I don't keep all my email in one global folder I have to select the different main folders for each account).This review is for a previous version of the add-on (0.3.5). This user has a previous review of this add-on.
You could just select the single 'Account Folder', the parent of the account's main folders (Inbox, Sent, etc.) - that works too.
As for a button - if every extension put its options in a button you would have endless clutter...