Well let's look at people's impressions of Dedede so far:
He's a very slow moving character on the ground, but his attacks are surprisingly quick, his slowest being his smashes. He has a very good recovery, a projectile that's easy to spam (better than Peach's turnips!) he can suck his own items in and shoot them out at close range, and his swallow attack is quick (and like Kirby's is basically an extended grab). He hits harder than Bowser.
Now any slow, heavy character that has the ability to attack fast can be played fairly well. Just look at Gannondorf in Melee. He had some surprisingly fast attacks and was considered a good character.
Dedede seems like he'll be going down that road. He's slow, but not when it comes to attacks, and he has a few tricks to help him out and ranged moves to spam at people who try to run out of his range.
I think he'll be a decent character, no?