I really like Mars Edit a lot. Great interface, big time saver. I use it with WordPress.
However, I can't really use it (and justify buying it) because it doesn't have support for Ultimate Tag Warrior. I realize that you can't support every plugin out there, but UTW is by far the most popular plugin for tagging in WordPress. Unfortunately, WP doesn't have a built-in tagging mechanism so this plugin is the only (or at least the best and most popular) way to do it.
If I can't tag my posts, I can't use Mars Edit.