Very useful for the two accounts I have with support of the quotas. Just one thing : it is a pity you have to restart Thunderbird to free the status bar. I regret that for most of my mail accounts the server does not support quotas, so for them I shall have to content myself of the display of the used size next to the folders. One thing has to be said : the pop-up is no more a problem for a long time as you can choose if you want it or not.
This review is for a previous version of the add-on (0.3.7).