The rules are simple - the session is to 500, accumulated, and I have a checkpoint every 50 hits. I can go as fast or as slow as I like, and I can't let the shuttle touch the ceiling.
After the 500 counts I go freestyle and see how much I can go without the shuttle touching the floor - my best is 468 so far.
It's a fun way to train the reflexes and to test out how fast a racket can go (4U rackets have the noticeable edge here), as well as string testing (I currently am deciding between 3 strings - BG6, BG66UM, and NG98).
I'd supplement the juggling with wall hits, but that kills off shuttles really fast in exchange for a great control and reaction training. Also, there're no walls here I can practice on...