I have been using Zend Framework quite extensively during the last few years. On a recent project, we had 8-9 separate applications integrated into the a single website. Many of these applications used a different a databases (Oracle, MySql, etc.), so we had to set up Zend Framework to be able to connect to any database at any given time.
I have been using ExpressionEngine and Zend Framework on my current project. We recently migrated the cms to Drupal (see migration), but we wanted to keep the Zend Framework apps. Most of the content of the website is handled by Drupal 7 (link), but certain urls on the website are handled by Zend Framework. On the Zend pages, I removed all layout information so that it is controlled by the Drupal theme.