I would advise checking out the problems at http://www.4clojure.com/ over the koans. You will definitely find some challenging problems. The nice thing about these is that you can look at how other users all solving the problems, which helps to convey a sense of style. I haven't done a problem in awhile, but I oftentimes gotten something out of looking at solutions from 'amalloy' and 'chouser' in particular. Of course, you can also check out my own ('gajomi') solutions :).