Fox: 3 words: TOO FREAKIN' FAST
Ness:His B moves are a little hard to pull off. Generally, if you can't use him near perfect, he's pretty difficult to win with
Donkey Kong: Terrible vertical recovery, but his moves ARE quite powerful and faster than Bowser, so, yeah, I use him occasionally.
Ice Climbers: Too light, and they're pretty much useless without the other.
Kirby: Besides Kirbycides in freindlies, I never really use this guy.
Link: I never really use his B moves besides UpB, but besides the fact he basically runs the same speed that he walks, I'm relatively good with him. In fact, he was the one I used to get the Master Hand Trophy.
Young link: Meh. To me, he doesn't seem to have as much killing potential as the old link, so, not really.
Marth: For the Obvious reasons.
Samus: 50-50 here. She's got awsome vertical recovery due to the bomb-jump, but you get knocked away too easily.
Zelda/Sheik: In all honesty, I never really use Zelda. Shiek is more Ninja-like, and we all know that Ninjas PWN. If your opponent is at least in the 30-50% range of damage, a sweetspotted UpSmash is almost a guaranteed KO. *Very useful when the opponent is falling right above you*
Pikachu: Really, besides Jigglypuff, I don't find the Pokemon characters all that useful.
Pichu: See above
Mewtwo: See above
Jigglypuff: See ab-oh, wait, nevermind. You know what's fun? Hitting someone with the Rest attack that they never knew existed XD
G&W: Okay most of you, you gotta admit this guy's almost always fun to use. Chair edgeguard FTW!
Roy: Eh, he just falls too fast. He seems too slow compared to Marth. He just controls....strangely.