Actually, I've given up on challenges / exchanges. I found them entirely frustrating, because you get or choose a prompt and a deadline that never gives enough time to research things, never accounts for all those beautiful plot bunnies that are sparked by - but unfortunately totally unrelated to - the initial prompt, and thus I end up writing a story out of necessity (because I promised) and not because I want to tell that tale at that point. It too often resulted in something I consider mediocre for my standards at best - and thus in something too close to cheap cheating. Since writing should be fun and not make you feel awful... :)