One of my side activities is managing the website of OrigamiUSA, which is based on the Drupal content management system. Earlier this year, we upgraded the site from Drupal 6 to Drupal 7, a process that was not lacking in adventures. At our local Tri-Valley Drupal Users Group meeting, I presented "Lessons Learned in a D6->D7 Upgrade". Here's the slides from that presentation.
[Update 2015-09-20] Having now managed OrigamiUSA.org in its Drupal incarnations for several years, I decided to convert my own site to Drupal 7 (although I shudder at the horrors that await in upgrading to Drupal 8). If you're reading this now, you're being served by Drupal!