The title says all, and that should be all the incentive needed to buy it.
The original was released on the NES back in 1985 and it helped to bring Nintendo to dominating about 90% of the market share during the 8-bit generation. (Most of the rest was held by the Sega Master System, and the remaining sliver left was the Atari 7800.) It was an iconic game, and it proved, mostly to Americans after the video game crash of 1983, that gaming could still be a good thing. That, and it was just one fun game.
You probably know all this, though Mario is a little guy who jumps on platforms still and moving, knocks out his enemies, and traverses castles to save Princess Peach, even though she's usually in another one. It's set up so well that these simple concepts are made so much fun to play through--timing right, trying to make that jump, and so on. Though again, you know all this, and thankfully, the classic is exactly as it was before--unchanged, unmodified, untampered with. Due to the Game Boy Color's button placement, it can feel a lot like a portable NES without the huge game paks, and controlling Mario is no issue.
Then, with cartridge space left, they decided to stuff the thing with plenty of worthwhile extras. The one you'll be playing longest will be Challenge mode. Here you have the ability to play through each and every level for 5 red coins, a high score, and a hidden Yoshi egg. If you've played any of the other Mario games involving red coin challenges, then this is no different. 5 red coins are placed around the level in some pretty clever spots that you have to find to get the challenge marked completed. High score is self-explanatory, and you must beat the given high score for that level to beat that challenge. Finally, the Yoshi egg challenge is actually where you will have to find a hidden, invisible block that you must hit from underneath to make the Yoshi egg appear. It sounds tough, though another extra was included to assist you in finding the eggs.
This extra brings up a spot where it will randomly select a level and allow you to see where it is, usually with a bit of the environment shown to hint it a bit. Others are a bit on the novelty side, like a fortune-telling one, and one where you can put your own text in any of the images provided to be shown before the title screen. There is also the Boo races, which gives you your choice of course and difficulty of the Boo opponent in a race to the flag. These are courses designed for the races and there are about 8 of them, using elements from the actual game in them. The whole thing wears off a little faster than I expected, though it isn't too bad either.
If you have a Game Boy Color or Game Boy Advance, go find a copy and enjoy yourself. It is a great time-waster on those long, arduous trips and it's easily one of the best games on either of the two consoles.