Very good

Solid MD editor with few distractions. It’s my go-to Markdown editor on both my Mac and iPhone.

It would be nice to have better custom CSS support and better previewing ability. When I do use the option to for a style-sheet, the in-editor “browser" does not seem to do a very good job of rendering the page—I still have to open in a web browser.

Update: I use Byword in conjunction with Marked 2 to alleviate the preview issue.

chigh about Byword