Second only to The Legend of Zelda as the "greatest Nintendo game ever" this game is a perfect rendition of the original NES classic, in the palm of your hand. Great for on the bus, showing off to people, and inciting spontanteous goomba-stomping matches with all your buddies that remember the game from long ago. Seriously, unless you still have your old NES out and it still works, this game is a great add to your gameboy library. Sure, the graphics and sound effects are from 1985, but that's part of the point. If you've never beaten this classic, here's your chance.
You can take it anywhere, play 2 players with a pal, link to another gameboy advance for 2P, and other couple of tidbits they don't tell you in the manual: After linking to another unit, you can choose to end the game and the entire game will stay loaded in the memory of the "client" GBA until you turn it off. Also, it works with the GBA wireless adapters if you've got them. Finally, it looks fantastic on the GBA's screen, especially on the "classic" GBA (not the SP) provided you can get the lighting conditions right.