Typically, I don't use Game Shark to help me win, but I do use it for novelty things. For example, I wanted to go through Crystal with an all-Dark-type team, just for kicks. I do that sort of thing; when I've already beaten a game, I play through it again and do a theme, deliberately limiting myself to add to the challenge. It keeps games fresh far longer than they would be otherwise.
Anyway, back on-topic, what Dark-types are there in Crystal for my all-Dark-type game? Let's see...
Murkrow: In Kanto only
Houndour: In Kanto only
Sneasel: not available until after more than half the Johto Badges
Tyranitar: not available until after ALL of the Badges
Umbreon: not available until Goldenrod, and since you can't fight wild pokémon in Goldenrod, it'll take either a very long time or more money than I have at that time to evolve it
This was a game where I Sharked a few species into the grass of Route 29 and caught them there at level 2ish.
Now, the Dark-type game was fun, but it's not something I would bring to a tournament. The only way I would ever use Game Shark on something destined for tournament battle is to make easier something that was possible anyway, like Sharking a certain Nature onto a pokémon rather than catching PC-fuls of them until I found one with the right Nature.