[eluser]Mirage[/eluser]
You can view Zend as a lower level framework compared to CI. A framework is little more than a collection of re-usable classes that help you reduced the amount of code you need to write to implement a given [Web] Application.
CI offers you a more complete workflow within the framework, such as URI routing and clear layout of controllers, models and views. That's what makes it easy to get started with and makes you extremely productive right off the bat.
ZF is growing nicely and very well supported. It's much more comprehensive with it's components than CI. But you need to ask yourself if you really need all of that (right now) at the expense of a longer learning curve.
I've used both and in fact ported ZendAuth and ZendAcl to CI libraries because I love how they work. But I still choose CI over anything else because it allows me to get things don't flexibly and most importantly: fast!
HTH
