A while ago (November 2012 to be exact), we upgraded the support.mozilla.org databases from Percona 5.1 to MariaDB 5.5 (the next step, happening soon, is upgrading them to Oracle’s MySQL 5.6). One of the engineers and I had a conversation where he mentioned that one of our worst performing views on SUMO is doing waaaayyy better with the upgraded databases, that it seems more stable and that I stopped receiving MySQL went away or disconnected emails which came in once in a while.
Its always nice to see upgrades actually making a difference. In our case we saw a lot less CPU wait, though that might also be partially due to tuning the memory settings on the machines and adding in another read slave to handle queries. As a result, network traffic throughput went from less than 1 Mb/sec to about 18 Mb/sec, because the machines were just handling more queries per second, period.
(I had this e-mail as a draft for a while and decided to clean it up and publish it now!)