I'll be posting my experiences in upgrading to and extending Drupal 7 both here and on my blog at Midwestern Mac, LLC (see D7 stories).
Congratulations to the team of almost 1,000 developers who helped make Drupal 7 a reality, and congratulations to Dries Buytaert, the founder of Drupal, and webchick, the person who shepherded (and continues herding) the community as the Drupal 7 core maintainer!
This website is still running on Drupal 6 (on a multi-site installation with about 5 other sites), but I'm slowly beginning the process of redesigning and upgrading the rest of my sites (notably, so far, Midwestern Mac, LLC) to Drupal 7. The Archdiocesan website and St. Louis Review will take a bit longer, since there's a lot of custom code that needs to be refactored.
If you run a website, have you checked out Drupal before? It's a lot more extensible (in my experience) than Joomla or Wordpress, the two other top contenders. If it's good enough for large sites like the White House and Examiner.com, it's good enough for you ;-)