Use CodeIgniter if you need PHP4. The two are similarly flexible and light.
Positives: * cascading filesystem with built-in support for modules * very clean and easy to use ORM implementation * auto loading of classes * easy to understand syntax * HMVC support is currently in development * open communication between community and devs * flexibility ... you can pretty much do whatever you want with custom class libraries, vendors without much hassle.
Negatives: * still a relatively new framework that is changing fast, but the current trunk branch is now stable. * expanded documentation module is in development
Kohana really does look like a great framework, however after reading the forums and wiki a little I'm worried that if I begin learning now, I'll need to change a lot of what I learn in the coming months with the 2.3/3.0 branch (whatever they end up calling it) - and whatever I start building will have to change too.
This isn't a huge problem as whatever I build now will be mainly side-projects which are designed to push my learning forward (such as small e-commerce sites, rating sites, file uploader sites), however considering the changes coming soon (API etc) it's a little frustrating.
However, thanks to you two I've found Kohana which is absolutely what I was looking for. Cake is a little too bloated and complex, CI is more flexible and simple, however to appeal to a larger userbase must support also PHP4 and 5, while Kohana is very niche insofar that is all about 5 and thus very much less restricted.
I'd be interested to see what other people who have worked around these frameworks think of Kohana.
I built a MVC framework for my project using their style and ended up learning a lot from them.
Also, about it being a new framework; the last I checked they were making a big change to the routing methods and folder structure.