The phrase that describes this game: "RUN FOR YOUR LIFE!!!!!!!!!!!!!!!!!!"
While Sonic Advance focused on bringing the original 2D games back to life (and it did with some success), Sonic Advance 2 puts sole focus on SPEED! PURE SPEED! The game has captured the essence of speed so much that you can blast through the zones and make good use of different gimmicks like ramps. Also, they decided to add a trick system that allows mid-air attacks as well as air boosts in case you're JUST about to miss that platform. The bosses even take advantage of speed, you'll have to be running pretty fast to give Eggman damage this time. The bosses are also quite challenging. They can get annoying and one of them can kill you INSTANTLY. YIPES!!!
As well, Sonic gets another character to add to its already decent cast... Cream the Rabbit. She's like Tails in the sense that she can fly (for about 1/2 the time Tails can). But the biggest plus is that she makes the game EASY (using her cute Chao pal, Cheese, to attack for her). This even works on bosses, so she can clear the game easy.
What makes the game hard is this: the Chaos Emeralds. To even gain entrance into the Special Stage, you need 7 Sp. Rings, which are hidden quite well in the acts. These are VERY hard to find and you need all 7 in one life. And it's even harder with Sonic, a hedgehog with no special abilities to fly/glide/etc.
Outside of things that make this game harder, Sonic Advance 2 is still a great game. With an excellent sense of speed, it blasts the competition, as is definitely one of the better GBA games as well as a better Sonic game in the series. Think Sonic's too slow? Wait until you play this... you'll think twice before you EVER say that again!