I like it, there is a lot of cool stuff there. At one point I messed around with it to the point where I had the basic functionality working and I thought it had the same feel to it as CodeIgniter. Though the documentation is lacking, it is really very simple. I feel good about investing the time into it because Zend is behind the effort. It is also very modular so putting together the Zend pieces is like building a custom framework and I felt like I had a better grasp of the internals this way. I also like that it is all PHP 5.
That said, I am still using and plan to continue using CodeIgniter because I know the framework and I know it gets the job done. When I have to get something done right now, I stick what I can get the job accomplished with… right now. CodeIgniter also has a great company and great developers behind it.
The Zend Framework is a framework in the widest sense. I don’t think it was meant to be a complete application like CodeIgniter. There are a ton of libraries that can really be used anywhere. I am sure you can take what you like and adapt it to CodeIgniter. If you want a complete functioning framework out of the box then do some SE searching, there are frameworks out there based on the Zend library.