Frankly I love it. Compared to other PHP frameworks I've used I think CI's way of doing things is pretty straight forward (but I suspect that Symfony comes close). Although I some qualms on how it uses validations, how CI's model implementation is different from Rails(personally it's not as magical as Rails implementation but I dig CI's more).

So I decided to tinker with the framework and fork my own implementation.

My framework is now built on top of CI and is able to use functional programming idioms such as anonymous functions and the like. Pretty awesome stuff.