If you like the Sonic series and the Smash Bros series, you've found a great game to play.
The story takes place sometime after Sonic Heroes does. It is a small story, but it's pretty good for a Sonic game. During the story, you get to play as every one of the characters- Sonic, Tails, Knuckles, Amy, Cream, and even two that haven't been seen on the GBA before: Shadow and Rouge. Every character's story ties together with the others, and Emerl(the robot)'s story is the final one.
Let me explain more about the robot- Emerl can obtain data about other character's moves during the story. It says that a light of some sort comes on when Emerl learns a move, but I haven't seen that happen yet. You can customize Emerl's moves using skill points which you can obtain throughout the game. You obtain 500 skill points in total.
The battle mode is one of the areas where the game really shines- with a few friends and some link cables, it can be almost as insane and fun as Smash Bros Melee!
The controls for the game are very smooth. If you've played a fighting game before, you'll get them quick. If you've played a fighting game and a Sonic game, you'll probably get them even quicker.
The battlefields in the game are 3D, but really don't have much variation. The sprites, however, are great and look nice.
The challenge mode is also a fun thing to do, and differs by character.
OVERALL: 8.5/10
This game is great! If SEGA ever made a sequel, I hope they'd put it on the DS. That would really help the game out.