First time I was exposed to the Game Genie was when @WhiteboySlim showed it to me. It was a gold half cart that connected to the end of any Nintendo game. After you switched on the Nintendo a screen came up to enter a code and after the code was entered you could start the game. Game Genie came with a manual for how to essentially mod or cheat at a game. The only mod I remember was in Super Mario Brothers...

! comment I saw on the Game Genie commercial "For rich kids who sucked at video games
I hit paydirt with one. I managed to edit a known code for Ninja Gaiden to basically plow through everything without trying. Essentially, you stand next to something, and it's dead.
I loved the Game Genie. Not for the cheat factor, but because I felt like I was doing something that I shouldn't be able to. I WAS HACKING THE GAME.
I loved it for the weird stuff: playing as the bad guys, replacing all of one sprite with another etc. You were able to actually make Shining Force II (my favorite game) a two player game, although with real intelligence, the enemy pretty much always won.
I agree, the Game Genie was game modding / hacking! Never hit as much pay dirt as your guys, then again I never owned one just played it at other peoples houses.
Playing as bad guys sounds awesome, that kind of rings a bell but nothing comes to mind immediately. Would be kind of interesting to play Mario as a Goomba!
I loved the Game Genie! I'm happy that I can still use one in an emulator, that and Pro Action Replay. I always found it more fun to play a game where I can't die, I know it sounds lame, but I just enjoyed many games much better that way.