I tried various Markdown editors (and paid for some) and I use Caret for writing blog’s posts. It is very convenient with autocompletion.
When you type **, Caret shows automatically **(caret)** so you don’t have to close the **. It’s the same for [, (, _ and so on - to get out of the block created, TAB will make the deal. It speeds my workflow. When I try other Markdown editors where I have to close Markdown syntax, I’m quite bored.
Why am I searching another editor ? Because Caret is not perfect. It has some visual glitches and don’t manage very well big documents.