And that's the winner in my book - this is not about doing a test or challenge once, and passing some "exam". Its about stretching me and discovering more about the strengths and weaknesses of certain languages and environments - I should do more little practises like this, but in different ways.

Same here. For that goal, PE is a fun and efficient playground. The forum discussions (only accessible after submitting the correct answer - always a number) are also a great way to see how the problem can be solved in other languages and programming paradigms. APL definately scores points in Code Golf :-)

I also like them to keep myself familiar with languages I'm not currently using for real work. I find that syntax slips out of my head very quickly, but that even writing fizzbuzz keeps it from falling away.

I did that with Clojure and it was great. Possibly an unfair metric because it felt as if many of the questions were purposefully built for a functional language.