Just quick and short: No, I'm not going to use it, because it caused more problems then it solved when I tested it.
One thing that completely brought my servers to a halt was using a PHP script as an image embedded in a website (transparent image as counter beacon). Somehow mod_pagespeed managed to request the image over and over again (i.e. my own webserver IP appeared in the access.log requesting that file).
Result of this is
a) in that special case the webserver locked up with too many requests and
b) I can't be sure that any of my scripts work after installing the plugin. A counter will not be able to count if the requests to the script are not performed in the way originally intended (i.e. by the user's browser). It would take much more time figuring out all things that break compared to optimizing myself.