After spending a few years with WordPress, I decided to migrate to Drupal. I installed WordPress in December, 2007, and replaced it in February, 2011.
Existing users had to recover their passwords, since ther was no clean way to use WordPress-encrypted passwords on a Drupal site. I also had the site off-line for part of the day while I replaced the WordPress software with the Drupal software.
My main reason for the migration is Drupal's "book" feature. Drupal makes it easy to structure short articles into a long, hierarchically-structured narrative. This is essentially how I write books anyway. This makes it easier to present complex topics built from a series of short articles.