motorhaven - 3:28 pm on Aug 16, 2013 (gmt 0)
I use it but only for a limited set of rules such as removing excess white space and comments from HTML.
Most of the things it handles like image sprite I've manually done so there's no server overhead from mod_pagespeed doing it. To me it's worth the time spent to manually optimize.
If you're scoring 98/100 I wouldn't bother, that's damn good and don't think mod_pagespeed will improve it based on my experience with it (been using it for about a year and tested many settings).