What want kind of websites are you running? If you're running WordPress I think it's better to use these two plugins together, Autoptimize and WP Super Cache. I used to install Mod Pagespeed on my server while it helps but it breaks other scripts. I end up disabling it for many scripts because it doesn't work with mod_pagespeed enabled.
Anyways, I think those 2 plugins together work just as good or even better. Plus Varnish, it's get even more awesome.