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Pro Great Markdown support without being intrusive
Byword de-emphasizes the syntax itself, while emphasizing its effects. It appropriately adds style, like italic and bold, to text that's designated by markdown and dims the syntax so it does not getting in the way of comprehension. Additionally, there are commonly used hotkeys (⌘b, ⌘i, etc) that can be used to apply style without having to know the syntax or having to type it out each time.
Pro Extremely simple
Byword is as simple as Markdown editors can get. Even though Markdown itself is not hard, Byword offers assistance to make it even easier with very little configuration required.
The UI is minimalistic and un-intrusive, the syntax auto-completion is excellent, and there are multiple small but helpful features that make writing Markdown with Byword a breeze.
Pro Available for all Apple devices
Byword is available on a Mac, an iPhone and iPad.
Pro Syncs using iCloud and Dropbox
Documents can be synced using iCloud or Dropbox from within the editor for use across all devices.
Pro Direct publishing to web platforms
Byword allows users to publish their markdown files directly to web platforms such as Wordpress, Tumblr, Blogger and even Evernote.
Pro Word count support
Has a built-in word counter.
Pro Multiple export options
You can export to HTML, PDF, RTF.
Pro A dark theme is available
For people who enjoy working during the night, there's a dark theme available that reduces strain on the eyes.
Pro Includes features that speed up writing
For example, while authoring a bulleted list hitting return automatically prepares a new bulleted line
Pro Live update support
Byword has a "Preview Markdown" option where it does exactly what it promises. It immediately compiles and displays the rendered Markdown file, making it easy for the user to understand how they are actually formatting their document.
Pro Supports Multimarkdown
Byword supports all the table types in Multimarkdown including table captions. MathJax support is added via metadata at the beginning of file was well as support for various diagrams. Table of Contents is also supported.
Pro Supports iCloud Drive and Handoff
One of the few (well, Ok, only for far) that has been updated for Yosemite features. Jan 2015.
Pro Updated frequently
Byword gets frequent updates which add new features or fix existing bugs.
Con Publishing requires additional in-app purchase
The feature to publish Markdown files to web services costs and additional $5 of in-app purchase.
Con Limited customizability
One of the things that makes Byword so easy to use is the fact that it needs very little setup and option-tweaking. However, that severely limits the ability to customize the editor itself. For example, there are only two themes available, a light one and a dark one.