Be aware though that CI tends to be looked down upon in a similar way to how PHP itself is. I like it (and it has good docs), but if you use it be prepared to be the lowest of the low :)
I used to use codeigniter, but there are too many things that (in my opinion) should be done for you, that you have to do yourself.
Codeigniter is a great first framework also. Much easier to pick up than CakePHP, or Zend.
For me it's about getting shit done, I'm too busy to worry about being looked down on by Zend users :)
I would suggest usually using a framework when you can.
It does have an issue I find significant, which is the Views being a mix of logic and templating. I find this messy and contrary to the original MVC pattern, since it pushes too much presentation code to the controller (it has to decide what formats to present the data in, for example).
But with KOstache[1], a Kohana module, you get your real Views again, along with logicless maintainable Mustache templates. Win/win.