You can probably best answer your question for yourself by watching some of the video's of Sonic in the
Sonic Video Thread.
Everyone else has pretty much already nailed Sonic's movement properties. Although he doesn't move nearly as fast horizontally in the air, its not nearly as a big a deal as with the space animals because for Sonic, its not that he moves horribly horizontally like the melee space animals, but simply his ground running speed is so incredible and his air speed is not.
Even if Sonic is one of the faster fallers in Brawl, this fast-falling is nothing compared to brawl. Even Fox now can shoot three laser shots from a single short hop.
I don't understand your problems with the jumping properties. The first problem completely eludes me. Could you clarify it? By quickly do you mean you jump too early, or that you fail to short hop? The second problem seems to be more of an issue of getting the spacing right for your jumps, and again it seems like it would be difficult to miss this if you were short hopping, since you practically don't jump over anyone if you short hop with the space animals. (I just noticed you're also having trouble with Falco
n in addition to Falco. Sonic doesn't have Falcon's insane aerial horizontal speed so I don't think he'll have a similar problem. Though I again don't quite see the issue, if you're using any of Captain Falcon's Knee set-ups, I don't think he ever really jumps above where he'd intercept his opponent if you short hop.)
If short hopping is in fact your problem, I don't think you'll have a problem with sonic. I never had a problem at all short hopping with Sonic in the demo, whereas I have to put some concentration in to doing it to do it properly with melee Fox/Falco, leading me to believe that Sonic's window for releasing is greater Fox/Falco's window in melee.