It works well (Win 8.1; Thunderbird 45). The main flaw (for my use and setup) seems to be that if a folder is selected that it does not take the subfolders with it. (it could be given as an option to include the subfolders or not in the settings of the addon) If it did I would give it a *****.
Running Thunderbird 31.6.0 - Allows me to view all unread mail across all e-mail folders (my e-mails are heavily filtered into folders automatically) in a single viewing pane, and that's just for starters. What an amazing new dimension this adds to Thunderbird!
Good extension, can really help save time, but would be a great extension if the functionality to apply the setting to all subfolders was included and the folder tree was retained in the filtered results. My tree describes the folder completely, not the name of the subfolder itself.