Depends... are you willing to learn how to play as Sonic? He's not pick up and play like the majority of the other characters. However, playing (and winning) as Sonic could possibly make you a better play overall.
Try him, read the stickies for more info, and see what you think.
Oh yeah, my opinion: I honestly don't see Sonic as a good secondary unless you're truly willing to learn how to play as Sonic or your main is Samus, Yoshi, Ganondorf, Falcon, or Link. There are probably two or three more characters that fall under this catergory, but you get the idea.