Fred Wu recently posted some of his thoughts on using the Zend Framework as a developer in other frameworks looking to expand his knowledge.
When Zend Framework was first announced, the developers have promised us one thing: extreme simplicity. I was 'extremely' excited, but it turned out that it wasn't the case. Ironically, Zend Framework is the most difficult one I have come crossed so far. I mean, as complicated as Symfony is, it has brilliant documentation and an active community to back it up.
He also comments on the level of documentation the framework has ("the documentation is often outdated, sometimes inaccurate"), the Zend_Search_Lucene component, some of what he thinks are missing components that should be in a 1.5 release, and a last jab at the naming conventions.