I grabbed the 64AK4K67XPJ3 promo code. I'm a frequent Markdown user, primarily using Vim to edit my text.
Macchiato is a nice start, but I feel like it needs a fair bit of additional polish and consideration before being worth $20, especially in light of the other options mentioned in the comments. Here are a few things off the top of my head:
1. Links don't seem to be styled at all.
2. You're not parsing multi-paragraph list items properly: All subsequent paragraphs are displayed as code blocks.
3. I can't readily tell if lines are soft or hard wrapped, and the 73-character maximum text width isn't visibly demarcated, which feels a bit awkward. Especially in the full-screen mode.
4. I can't customize the styling at all, and I find Courier New and the gradient on the background to be unpleasantly distracting. I wouldn't be worried about this if the default aesthetics were more to my liking, but then again, you can't please everyone that way. :)
Since you're focusing on Lion, Menlo, introduced in 10.6, might be a more sensible font choice, and I think I'd prefer a lightly textured gradient-free background to the current styling.
5. I recognize that you view the unification of styled and plain text representations as a feature, but it personally doesn't work for me. When I write Markdown, I want to ensure that the plain text representation is legible and stands on its own, and I want to make sure that everything is being parsed correctly so that I can easily publish it for consumption by browsers. For me, mixing the two doesn't serve either purpose well.
For instance, it's difficult to tell if I've properly nested everything when I have an ordered list containing multiple paragraphs per item, possibly interspersed with code blocks and block quotes.
6. I somehow trigged a bug where my editing cursor appeared two lines above where text entry was actually happening.
7. It doesn't help me write Markdown, at all. A few places it would be nice:
7a. Hitting return after writing a list item. I'd like it if a new item was automatically created for me, with one backspace removing the new item but not de-indenting my cursor, and a second backspace actually de-indenting.
7b. Writing links would be easier if Macchiato matched braces for me, and possibly did some magic for [reference style][0] links.
7c. Writing code blocks would be easier if Macchiato maintained the same level of indentation after I hit return on each line, instead of needing me to tap out a bunch of spaces.
7d. I can't tell the difference between a space and a tab when I'm writing.
8. If you're limiting line length to 73 characters, why am I allowed to resize the editing window to be significantly wider than that?
There are plenty of nice touches: The support for auto saving / versions seems great, and I rather like the toggle-able word/character count in the title bar. The core of a great product is definitely there, but it needs some more cultivation.
Hopefully this is helpful and not discouraging! I didn't want to take the promo code without leaving you with some feedback.
[0]: http://daringfireball.net/projects/markdown/syntax#link