It works just as well as the default editor when posting your comments, and has no loss of formatting. Also, when you edit your posts, it doesn't add more spacing between lines with every edit, unlike the TPT editor. However, the page may take a few seconds longer to load when running this script, since it needs to fetch an external script for the editor.
Download the script here. You'll need Greasemonkey installed (or Tampermonkey), or some other userscript runner.
> wolfy1339's Powder Toy Enhancements script
Correct attribution would be nice... it's a very slightly modified copy of https://gist.github.com/jacksonmj/e2a5bbcd5b632f68c85f
(wolfy1339's version of it has formatting changes, a couple of extra bugs (in tptenhance.makeSaveLinks and tag moderation), some unused variables removed, and some variables made global for no apparent reason)
The current WYSIWYG editor is actually TinyMCE, just an old version of it (v3.5.0.1).
This script works in Chrome, but not in Greasemonkey in Firefox - the new editor appears but I can't type anything into it. Also, the save comment textbox probably shouldn't have a WYSIWYG editor, since HTML is not supported in comments.