I recently bought a couple of these flash carts for the Gameboy Color:
http://www.youtube.com/watch?v=E1ZWnA27FIw
I like my Bung GBX flash carts better, because they are 64Mb without having to flip between pages by powering the Gameboy on/off really quickly. However, the GBX carts require a parallel port and soldering in order to replace the battery used to keep saves. The GB USB Smart Card uses USB instead of parallel port and also has a battery that can be replaced without soldering.
The other disadvantage of the GB USB smart card is that it can only store a single save at a time, i.e., multiple games will overwrite eachother's saves.
http://www.youtube.com/watch?v=E1ZWnA27FIw
I like my Bung GBX flash carts better, because they are 64Mb without having to flip between pages by powering the Gameboy on/off really quickly. However, the GBX carts require a parallel port and soldering in order to replace the battery used to keep saves. The GB USB Smart Card uses USB instead of parallel port and also has a battery that can be replaced without soldering.
The other disadvantage of the GB USB smart card is that it can only store a single save at a time, i.e., multiple games will overwrite eachother's saves.