I've been putting it off for a few years, but I finally decided to upgrade devbee.com to Drupal 6.
I didn't really need to, but it bothered me that I wasn't running supported code and I figured I might learn something. And I did. Mostly obvious things that I should be familiar with already.
I've only ever played around with this. I don't like learning new things unless they are going to be truly useful to me. Drush is definitely something I shouldn't have ignored for so long. It comes in particularly handy when doing a site upgrade as you can download and install modules, clear cache, set variables, run DB updates and a lot more all from the command line. This tool is crazy good if you're comfortable in a terminal.