On 5/22/09, I upgraded to Drupal 5.18 (from 4.7). All went well. You'll notice changes as we continue to integrate the existing site with the new code base and the new theme. I will be rolling out additional improvements in the coming weeks. For now, you'll be happy to know we have a fully functional chat system as well as the Shoutbox (which has made it's glorious return).
If you notice anything broken or have questions or concerns about the upgrade, head on over to the Bugs and Feature Requests forum and let me know.