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Ness is hard to get used to but just like Mr Game and Watch, he will prove to be a very good character. I'm not actually very good with him on melee (I was quite good on SS64) but he's got some very effective moves.
Jigglypuff is generally useless, but, all because of his new B move on melee (the roll) I think he has become incredibly good. Using his roll alone, I've beaten 3 level 9's and even my group of hardcore friends a couple of times. Once I even rolled from almost the edge of the map on North Castle through the air all the way to the small platform below the castle.
The least playable in my opinion is Pichu. Why the hell would anybody use that t**t, who hurts himself with his own thunder attack, rather than the real deal Pikachu? Pichu's weaker too. He's the biggest waste of a character I reckon.

i play with all characters, mostly kirby, but the ones i usually pass by when picking would be dr. mario and peach. i don't care for peach and dr. mario is a copy of mario pretty much.

i dont think that any of the characters are not playable, some just take a little longer to master than others do

All characters can PWN (with patience and practice), it's just some have to struggle a lot more than others. Mewtwo is arguably the worst character in the game. Pichu has his evasiveness and Bowser has his weight and power, but Mewtwo really needs helps. He needs to be played flawlessly to stand a chace and even then it's an uphill struggle for him. Jiggs, Ice Climbers, and Peach don't suck. Peach is one of the best in the game and the others two are some of the better characters. *shudders at wall of pain, Peach's turnip, aerials, and down smash, and dysynching*
Here are a some good vids of pros.

http://www.ssbm.it/awesomevs.htm

http://www.ssbm.it/musicvids/Pure_Nastyness.wmv

http://www.ssbm.it/musicvids/ShinedBlind.wmv

http://www.ssbm.it/musicvids/The_Hylian_Nightmare.wmv

http://www.ssbm.it/musicvids/Death_From_Above.wmv

http://www.ssbm.it/musicvids/Overdose.wmv

http://www.ssbm.it/musicvids/All_Wessed_Up.wmv

I think I'm pretty good with most of the characters. I find Fox quite difficult though because he's too fast to control and when I try and do his B+Up move I accidently use B+Left or Right which sends me of the edges. Also I'm much more used to using Falco so it's weird trying to do Fox for me

Character mastery times range from your first battle (Shiek) to months (fox). Sheik is so easy to use that people have stopped using him/her out of boredom.

Pichu is a waste of a character, and Mewtwo was toned down too much to be a good character, but the rest are good.

There's nothing wrong with Mewtwo and Pichu when you think about, but they have nothing going for them or anything right when you compare them to characters like Sheik and Marth who I consider near broken. There are few characters who can go toe to toe with those two. In a game with 26 characters (Sheik and Zelda are are too different and played way differently to be considered one) that's a problem in terms of balance. Pichu self damage isn't his problem, it's his lack of range, power, weight, knockback, and priority. Most of that is also true for Mewtwo. Both of those characters have to be played flawless or near flawless at their metagame to compete and it's still a difficult struggle for them.