I would recommend the Zend Framework above all other PHP frameworks.
I have really enjoyed working with the Zend Framework. There is a lot of support behind it and it's actively under development. They are practically shoveling out new features. Now with version 1.8, there are more features than ever. The MVC components alone are worth it. It has a ton of APIs and is integrated with Google, Amazon, Yahoo, and Flickr APIs, among others. Working with databases is a breeze. So is user management/user authentication. It is amazingly flexible. There is no configuration required to start using it. Just drop the directory in your project and start using it. You can include individual classes, or you can "bootstrap" your application and "autoload" all the Zend Framework classes (and your own). Also, the Zend Framework is continuously tested for bugs. Since it has a suite of unit tests, it makes it a RELIABLE framework to use. You can trust that it will work the way it's supposed to.
There is a ton of official documentation, though not always the easiest way to find out how to use something. Often I just search Google for tutorials that people have posted. There are a lot of them! Also, words of warning...since the Zend Framework has evolved so rapidly, a lot of the books/tutorials out on the web have become outdated. There aren't many books available that are up to date (especially now that 1.8 has introduced the Zend command line tool, adding a new way to approach starting a new project).
Also, I think learning the Zend Framework is a valuable skill. There are many new open source projects that are being built on the Zend Framework (Magento for example). Employers are also looking for people who have worked with the Zend Framework.
The Zend Framework is not perfect. There will be times when you become frustrated with it, but from my experience, there are lots of resources available to help you when you need it, and the Zend Framework is the way to go.