I'm still new at this. I've only unlocked a few characters (Ness, Marth, Luigi, Falco, Snake), I don't have all the stages, and I haven't even touched the GameCube controller yet. But this is just my first impressions of the game.
Some things you should know about the characters:
Link: Amazing, as usual.
Sheik: Even faster than before.
Yoshi: More terrible than ever. (I don't like Yoshi.)
Pit: My favorite newcomer. He's like Link with wings. Amazing!
Wario: Overall a pretty okay character. He has weak attacks, but he's funny to watch.
Ike: Very slow. Mediocre attacks. I suggest standing in the middle of a stage and waving your sword around. It's the only way you'll get a KO.
Pokemon Trainer: Hard to get used to, but once you do your opponents better watch out.
Diddy Kong: He's like a shorter hairier version of Link. Plus he has funny facial expressions which is an added bonus.
King Dedede: Just an obese Kirby clone.
Snake: Slow-moving. Attacks are hard to get used to, but they make up for themselves with their amazing power.
Luigi: Not as much of a Mario clone as he used to be.
Controller
Like I said above I haven't touched my GC controller since I got the game. Just doesn't look fun to me. I want to experience something new. They're all really hard to get used to.
If you're an old school gamer like me you'll really like the horizontal Wii-Mote.
If you picked up gaming around the Nintendo 64 era you'll like the Classic Controller. It's very similar to the N64 controller.
If you just picked up gaming you'll probably want to go with the Wii-Mote and Nunchuk. It's the easiest to get used to and let's face it- it's loads of fun to be able to flail your arms around freely as you play.