As no-one has voted for Symfony here i will and here's why. There are two types of frameworks, well a whole range actually but in the PHP/MVC area which is where we are in this thread there are Glue and Full Stack frameworks.
Zend and CodeIgniter are Glue and Symfony and Cake are FullStack.
Glue are the ones where you can pick and choose which components you can use and how much "standard" code you can use. These tend to have a gentler learning curve as you can pick the bits you like that help and fill in the more difficult bits with code you know.
FullStack means you need to use the lot and so the learning curve can be quite steep. Also with FullStack there can be a tendency to balk against the way something is implemented rather than just accept and flow with it.
Coming from a write everything myself background i initially favoured the Glue's but have now migrated to FullStack using Symfony 1.4 and 2 and Sinatra and Padrino. The extra speed and power the fullstacks give is not something i would like to give up.
One downside of CI is that it is built for php4 compatibility and so does suffer in a number of structural ways when you come to push the framework, Kohana is a fork that addresses this issue. And i dislike Zend because there are so many ways of doing the same thing that after a while the Framework seems almost irrelevant (Sorry personal rant)
At the end of the day use of a Framework is good because it adds a structure and can be a great aid to learning and the one to choose is the one you feel comfortable and are productive in.