The idea of having a memory card that can also douple up as a mini game itself is very clever.  The Vmu will save you data and also gives you the facility to exchange data with a friend by conecting the two VMU's together.  The VMU is also a clock, the VMU also turns into a miny game.  The mini game works depending on what information is saved.  For instance if you had power stone and complete the game with different characters you are awarded a prize, one of the prizes is a mini fruit machine to play on the VMU.  If you had sonic you can gain a little cyber pet type creature which you raise on the VMU to help your progress on the actual game.  So overall SEGA have come up with one fancy gadget