Fantastic for Adults
If you are from the "old" NES school (like myself), you will love the mini-games associated with the character "9-Volt". There are mini games based upon such classics as the original Legend of Zelda, Donkey Kong, Hogan's Alley, Duck Hunt, and many more!
Once you unlock a game, you can play that one game as many times as you can in a row. Although this sounds like it should be easy, the game speeds up the action and it becomes very challenging and fun. I've been playing my GBA to kill time waiting in line, because the game is not linear in the traditional fashion.
Enjoyable, challenging, and non-traditional. All great qualities, especially at a time when the best GBA games are primarily re-releases of SNES titles!