Sonic is one of the fastest characters in the game, with his high movement and attack speed giving him a strong rushdown game. Sonic can use his Spin Dash to approach whilst dealing damage, and he has some decent finishers. He can use his Spin Kick to overwhelm his opponent, then KO them with his Flash Kick. This makes Sonic one of the better beginner characters to begin with.
However, Sonic has range problems, which gives him a very difficult time against characters that out range him. He is also totally incapable of handling projectiles, as he lacks a projectile himself. Sonic's movement speed actually works against him, as the game's physics make him very hard to control. Sonic lacks survivability, as his jump height is only average at best, he cannot momentum cancel, and he has a difficult time countering opponents' edgeguards. These traits give him bad match ups with higher tiered characters such as Lloyd and Tails.
Sonic is ranked multiple places below Super Sonic as the latter is much faster and has better finishers than Sonic, despite a slower attack speed and a larger hurtbox. This makes Sonic pretty much obsolete in the convoluted environment of SSF.