A current project we're working on at Chocolate Lily involves upgrading a Drupal 6 site to use the Drupal 7 Open Outreach distribution. It's the first time we've used a Drupal distribution for an existing Drupal site instead of a new one. Having worked on my share of hairy Drupal upgrade projects, I went into this project with a fair bit of apprehension.
But I've been pleasantly surprised. The basic upgrade process was relatively quick and painless.