I'm a little late to this party but last week Brett Terpstra released version 2 of his Marked application. Marked is the last word in markdown preview on the Mac. I've already purchased mine and am using it.
Marked 2 tracks your file changes and updates the preview as you go, even automatically scrolling to the point in the file where you're currently working. The new version is faster and more accurate. I know there is a lot of work behind that speed improvement.
The application also supports MultiMarkdown 4.2 (including footnotes), better search, Fountain support, MarsEdit and system clipboard previews. You can also export to DOC, DOCX, ODT, RTF, RTFD, and paginated PDF. This is just a taste. Brett's gone crazy with great features. If you routinely write in Markdown, you'll have use for Marked 2. Get it from Brett directly.