A refreshingly simple board game that is all about spacial awareness, "Callisto" is enough fun to pass some time with, and a welcome break from the overly complex, electronic games that have come to dominate the market. The game is flexible to enough to work well with 2-4 players, and one nice touch is that the playing area can be changed depending on the number of players; so the challenge remains relevant to the numbers involved.
Apart from that, it's pretty much a matter of being good at getting rid of your collection of assorted shaped tiles within an agreed playing area of the board. It's engaging enough that on opening the game for the first time, my family and I played 4 games straight off.
As with most games that last, it's the simple design that will keep you coming back for more. Certainly fun, and makes you think.