Yeah, he's really bad. He has ZERO priority, no kill moves, and can be killed at less than normal percents. All he has to KO are his f smash (they still have to be at like 120% + for this to kill) and his b air (which is only good when they are near or off the edge). His d smash is decent too, but they have to be at high percents for it to kill.
What I do with Sonic is run around, looking for an opening to attack (I can't just run up to them because he has NO priority), try to get a hit in and then chain together some more hits. Sonic is so fast that, if you read your opponents DI and dodging correctly, you can get several hits off of one. The bad news is that he's very easy to deal with if you become predictable. That's the downfall of most Sonics. If they know what you're doing, you're screwed.
All the top and high tier characters just **** Sonic. ****ing DDD can CG him and kill him at medium percents and Sonic can't do **** about it cause you can't kill that fat **** until he's around 180-200%. Don't even get me started on MK, his up b just destroys Sonic at low percents. I actually enjoy playing Snake with Sonic, although it's also a hard matchup. The mines and tilts really kill Sonic fast. You have to dodge things like CRAZY to beat a snake.