Talk about a gimmick to get people to spend a few bucks more on their kids. Nintendo had released this Gamecube to GBA cable years ago. Its sole purpose is to allow you to hook up your GBA to your GameCube, but only for specific games like Pokemon, Fire Emblem, Zelda, Mega Man, or Final Fantasy Crystal Chronicles.
Outside of your kids using it a couple of times, this thing will probably just be cast aside in the dust or lost because you really get little use out of it which is why I think buying the official Nintendo brand cable is ridiculous for like $10 or so. Might as well get a second rate third party one for a couple of dollars.
What kind of stuff does it do specifically? Well in games like Zelda it gives you a mini-map which you can look over on your GBA screen. In MegaMan X COmmand Mission it gives you access to hidden items. In Fire Emblem you can get new items and unlockables through linking the 2 systems. Its the only method of multiplayer for Crystal Chronicles - so unless you're a huge fan of that games series and have a group of friends or siblings to play alongside you, then these cables really won't get any use at all. At best you'll probably only get around an hour's use out of it ever year if you're still playing those GameCube games.
The product works, but I think Nintendo, while its a great company with amazing games, sometimes takes advantage of their customers by making us dish out unncessary cash to enjoy some of the games completely.