I've never used ZF as a full stack framework, exactly because of its complexity, but I've plenty times used selected components from it. It makes a very fine component library in that sense - I just hope they don't mess that up with ZF2.
Agreed, I've done the same. That's also why I don't shy away from the idea of rolling my own framework. I've had to take some ZF components, like Zend_PDF and Zend_Mail, and add them to my own code before. It's easy, and obviously relieves the stress of having to roll such components on my own.