Well, this game is great. It implies much more strategy than it's older brother on the N64, but lacks in the same fun.
In DD (double dash), you will be using 2 characters at a time from the Toadstool Kingdom. They still have the limitations and advances with weight, so having a weight of one character different from the other will alter the over all weight, which will decide what weight class kart you get to choose from.
You have the same items as from the first Mario Kart, with a few new ones that are known as "special" items. Each character has his/her own special item to use, all of which are quite powerful when used correctly.
While the game delivers fun on many levels, it just seems that you'll be missing the N64 one while playing the GCN sequel.