I use this for about two thirds of my outgoing emails to get a neat and professional look. It's easy to use. Anything more than just a few words, I compose my emails in my text editor and paste them over to Thunderbird, then hit the conversion button, and if I'm happy with the look, I send. This workflow of mine avoids any glitches between Thunderbird's somewhat mysterious WYSIWYG-ish text formatting for outgoing stuff and Markdown Here's two different states (applied, and not-applied).
This review is for a previous version of the add-on (2.13.2).