Back in August 2013, I wrote the original version of this article on my Drupal Gardens blog.
Whether you are just moving to Drupal or upgrading to a new version of Drupal, if you are starting with an existing website, you are facing the same problem: Your migration timeline.
Drupal.org provides a number of pre-packaged distributions (e.g., Drupal Commons, DKAN, etc.) that allow users get a fully-featured Drupal installation up and running in no time, but maintaining an installed distribution can be tricky. You may need to juggle distribution updates with contrib module updates, core updates, and your own customizations. If you aren't careful, it can be come a maintenance nightmare!
As Dries has stated many times, Drupal is the future of the web, and distributions are critical to that future.