In a word, excellent. Game Genie was a line of cheat devices for multiple home and portable console systems, designed by Codemasters, and marketed by Camerica and Galoob.
Ever doing what Nintendon't, the Sega Genesis Game Genie featured a switch to enable and disable code functions during gameplay. In addition, this version was endorsed by Sega, being officially licensed. A side effect of the shape of the cartridge is that imported games can be played on the Genesis/Mega Drive it is plugged into, provided it is in the same television region.
Sega Game Gear
The purpose of the clear window is so that one may attach Game Genie codes to the back of the Game Gear cartridges, and read them while on the code entry screen.