My company's big project right now is built in CodeIgniter. We picked a framework for a number of reasons, but mostly as a way of standardizing the code. Rather than everyone having some arbitrary way of doing something, we found it easier to use a set framework with its own conventions, at least in terms of naming schemes and file structure and whatnot.
We went with CodeIgniter on an outsider's suggestion, and in retrospect, it was probably not a great choice. We've found a few weird things in their core classes that we've had to modify, which should never happen. I've used Zend since that project and I greatly prefer it over CI. The documentation can be a little funny (explanations of functions are in places where you wouldn't expect it) but overall I've just found it to be much more robust.
I've heard good things about Kohana, so I think if I were to start a new project, I'd look into it. I think my biggest concern would be being able to update the core classes without having to rewrite all the code, so what ErrantX said here about Kohana going from MVC to HMVC is sort of worrisome.
We went with CodeIgniter on an outsider's suggestion, and in retrospect, it was probably not a great choice. We've found a few weird things in their core classes that we've had to modify, which should never happen. I've used Zend since that project and I greatly prefer it over CI. The documentation can be a little funny (explanations of functions are in places where you wouldn't expect it) but overall I've just found it to be much more robust.
I've heard good things about Kohana, so I think if I were to start a new project, I'd look into it. I think my biggest concern would be being able to update the core classes without having to rewrite all the code, so what ErrantX said here about Kohana going from MVC to HMVC is sort of worrisome.