The game is seriously worth it just for having 12 racers at once, and the online capability. Double Dash got a little dull once you could pwn the computer players (The AI seems a little better in this one, but not much). The next iteration of the series just needs to include every track ever made, and the option to go use a DD kart with a friend riding shotgun.
If you have a Wii, you need this game. If you don't have a Wii, then I don't know if any argument will work to make you get one, as this game, Smash and Galaxy are pretty much the only ones worth getting.