According to Google, over 120,000 sites are currently using mod_paegspeed to improve their performance, including hosting providers such as DreamHost and Go Daddy, as well as content delivery networks like Edgecast.
If you're not focused on speed and performance of your Web property, you most definitely should be. Google hasn't hidden its preference for faster sites and even goes as far at to indicate that faster pages do lead to higher user engagement, conversions and retention - in fact, speed is one of the (many) signals in search ranking and ad quality scores.
Even if you're not about to go through the rigmarole of implementing mod_Pagespeed, it's still important to focus on matters of performance. Beginner and intermediate users should focus on speed and can do so in the site speed reports available in Google Analytics. Users can (and probably should) also explore Google's other speed and performance offerings including PageSpeed Insights and its PageSpeed optimization products.
Below is an hour-long video from Google with lead developer Joshua Marantzon discussing the official 1.0 release of mod_Pagespeed: