There are 20 different prizes, 12 of which I would be siked to get, 8 of which are useless to me. I have opened 22 boxes and recieved 19 useless prizes. I still like the event because there are now twice as many rooms up on Kaiser (like 5 or 6), and I have 3 prizes that I would have otherwise not recieved.
I just don't know why they do these "mystery box" events instead of just random drop events.
edit: up to 21 useless out of 24