Reading some of those reviews where it says "my save files were corrupt" and "it didn't work on all my games" tells me that they don't know how to use the gameshark.
There are some harsh realities about the Gameshark like: if you use the wrong code at the wrong time, you'll corrupt your save, or; you need to boot the Gameshark with certain games with work with other games to bypass lockout/anti-piracy stuff on the 64 and if you boot it into a game mode for a game that you don't have you CANNOT boot the Gameshark until you boot it with that game. That last one is confusing, I know. I can explain it better if you don't quite catch my drift.
Also, it's possible that the contacts get worn down as time goes one, meaning it'll take more work to get it to work the longer you own it.
It isn't an official Nintendo product after all...
But if you keep in mind what you're doing you won't corrupt your gamesaves, or lock it into the wrong game mode, and if you take care of it/be gentle with it you won't have too many connection issues. Basically, it's not as robust as a regular N64 cartridge.