When writing a comment with a lot of formatting, especially one containing formulas, I often found myself pasting it into the "new answer" editor at the bottom of the page to see if everything was rendering properly. But this is kind of tedious.
So, I've written a userscript that creates a new live preview directly beneath any comment you compose or edit. This script has made writing such comments a much nicer experience for me so I hope others enjoy it as well.
It works on all Stack Exchange sites (as far as I know) but I wrote it with Math SE in mind. MathJax is rendered in the new preview just like when you're writing an answer, if a little more slowly.
This script does not work in Firefox (yet?).