Sonic is terrible. Brawl has less hit stun than melee and it makes it hard to combo. Reduced hit stun makes it almost always unsafe to try to link attacks and gives a huge advantage to characters with higher priority and disjointed hitboxes.
Characters that have no projectile or disjointed hitbox get ***** by this, like Sonic. He's fast and is supposed to be able to combo, but he gets punished easily if he tries to. Keeping a projectile, sword or high priority attack out at all times makes things super hard for sonic.
Uair,UpB, Uair is not a combo. You might catch some people offguard, but they will be able attack or dodge before that second Uair comes out.
Despite this, I Sonic still seems usable with mind games and hit and run tactics.
lololololololololololololololol, someone's claiming they know metagame already
Sonic's uair is like Fox's old uair. Two hits, one of which has obscene priority, and seems to be a disjointed hitbox. Uair, UpB, Uair is veeeeeeerry situational, but works perfectly when you set it up, due to this.
Also, sonic can Uthrow double uair most larger characters at 0-10%, guaranteed. Peach is an exception, but at around.... 40% you can uthrow single uair her, tangent upon DI. If she goes behind you, bair, if forward, fair. Though really, if you anticipate the DI, you can follow and catch with the sideswipe of the uair.
Sonic can indeed combo, though he's no CF yet. Also, not all characters with disjointed hitboxes have the advantage against Sonic. GaW, D3, and Marth are all easily punishable the moment they take to the air, or do an attack. D3 is the exception, due to being able to hide behind waddle dees (to an extent), as well as CG Sonic for obscene damage. Ike is slightly different, in that once you get in, you can mess him up badly, but if he predicts you right 3-4 times, you're dead.
A majority of Sonic's approaches are ground based, and require either power shielding, or knowing when to spot dodge in his run/usmash/whatever. Still, once you've got the feel for it, he does relatively well.
Sonic is NOT godly, by any means, or possibly even good. Until there are more combos to link into his kill moves (jab cancelling into fsmash is meh at best, and it's rare that you can throw into bair, if it occurs at all), he's going to be very frustrating for anyone without a decent grasp on yomi layers to play as. Personally, I'm fine with this, since it means I won't have to worry about too many scrubs massing tournaments with Sonic, at least for now.
EDIT: Uair, UpB, Uair works mainly against space animals. They're still ******** easy to combo. Take it with a grain of salt, though. You have to set it up perfectly, or else they can get away. I only had like... 2 opportunities for it throughout a whole tournament. D: