I find both Sheik and Sonic to be at least playable. Falcon not so much. When he has a bad matchup, his weakness is VERY apparent. The other two, well, not as much. Sonic also has a ludicrous recovery, which in itself makes him leagues better than Falcon overall. He's an uncomfortably weird character, though, when it comes to playing him.
I find Yoshi, PT, and Samus to be contenders for the "Worse than Captain Falcon" award. I know Yoshi is probably misplaced there, but I have extreme difficulty playing him. Link may also deserve a spot somewhere in the lower tiers. It's hard to imagine someone who can be disgustingly outclassed the way Falcon can, though. Take Falcon vs Olimar, Ness, Kirby, and Lucas, for example.