If you have a younger child that wants to play (or you want something for them to do in the car) I would buy this for them over anything new. It doesn't cost much, there are hundreds if not thousands of games to play, and it's very simple game play. With the release of the DS and PSP the price of the GBC's will fall to their lowest, so if a child drops, breaks, or looses the system it's not a big loss.
I wouldn't recommend it for the average modern day gamer ages 12 and up unless they really enjoy the classics. There are no hook ups (except for a link cord if desired and a pug in to save the batteries) and the battery life in the system is excellent - the only complain I have with the system is that it does not have a back light (meaning there has to be a light source from else where to see the screen).