Just to throw a slightly dissenting perspective ... I don't know, I've been interested in the concept of katas in a sense. I've done them a few times, but I always seem to have another side project (or three) that I'd rather spend my extracurricular time on. I guess if you have nothing else to work on katas are fine, but otherwise, just pour your time into a side-project that you're passionate about :)

Of course, that's just my viewpoint ... I understand it won't work for everyone. So YMMV