The slim framework docs are perfect, the code is clear and the forums are great. I especially like that it doesn't force you to do everything in one predefined way, making the learning curve nice and low
The best parts about it are that the source is very readable and it's extremely easy to override the default behavior of many modules. I would strongly recommend doing this as much as you need to because Kohana's defaults may not make sense for your usecase. The biggest downside is that v3 is very different from v2 and a lot of the documentation and help still assumes v2 (this is why easy to read source code is important.
I am using it for my current project, I have only experienced it for about a month I think :)
If you want a full featured framework, Symfony2 is a good everything including the kitchen sink although it's a bit more Java-esque (which make sense since much of PHP's OO design is based on Java). http://symfony.com/
For something lighter weight, Silex is based on the Symfony2 components, so it's super easy to pull in more components as you need them. http://silex.sensiolabs.org/
Even if you're not using Symfony2 it's really modular and easy to integrate with other frameworks - we're running CI as our base framework due to legacy reasons, but all of our new work leans heavily on Symfony2 components.
I have used Cake PHP and Yii too. But they are not that modular and flexible when it comes to CI.