There's an obvious need for something like this. On the one hand, it's Markdown friendly and will integrate cleanly with many plain text workflows. On the other hand, I'm not sure I buy CriticMarkup as "readable in plain text". The syntax involves many characters (quantity-wise, not variety). The farther apart words are forced by syntax, the more difficult it becomes to read naturally.
It's not awful with the color-coded syntax highlighting, but would be much less readable in, say, an email to a colleague who has never heard of Markdown and the like.