I use FastMail and this add-on works great for composing in a MarkDown editor (WriteMonkey, MarkdownPad2, etc.) and then pasting in FM's compose window and toggling the Markdown into formatted text.
But there is a problem I see when I use numbered or bulleted lists. Markdown Here will properly indent the list. However, when I'm done with the list and wish to resume regular text UNindented, Markdown Here will not unindent the text and move it back out to the original margin, even though previews of the text in WriteMonkey and MarkdownPad2 will show the text following the lists as unindented.
Apart from this issue, Markdown Here has been working great for me.
EDIT: D'oh! Now I can't reproduce the formatting glitch I mentioned above. Markdown Here is working perfectly. Oh, well. I guess I'll have to live with it. ;-)
This review is for a previous version of the add-on (2.12.0).