I wouldn't say it's too reliable. GS allows you to do things that you game was never meant to do.
I had a N64 one. It was nice to unlock the wrestler in Revenge (I believe), who you couldn't get otherwise, and go across the dam in GoldenEye. For a while I was using it with a different save file, and messed around that way.
Then one day I was messing around on the 3rd Zelda save file. It was neat, some items appeared that were not in the game, and with Japanese text. So I played around a bit and then one of the times I booted the game, everything was gone. The first save file which I had completed the game, and the 3rd which I used for GS, all gone. Needless to say, I never used my GS after that for anything other than playing imports.
Is that reliable for you? You might get older Pokemon, you might lose all of your Pokemon. My suggestion is that you wait. Pokemon is a game of collecting, and once you have everything, there's no point to play it anymore. But if you really want those guys, I'd buy the other Pokemon version. Use your Gameshark on one version, and then trade them over, just in case anything goes wrong.