I waited a long time for this game to come out. Not only was it a decent time gap since Double Dash, but it was a killer-app for the Nintendo DS not only as one of the first awesome games, but the first to really take advantage of online play through Nintendo Wi-Fi connection.
Granted online play was slow to connect, but I had a lot of fun after the game was set-up. Then snakers cheated in every match a few months after the game came out, and it was no longer fun to play over the net.
I guess it's not that big of a loss. It has plenty to chew on in single-player, and offline multi-player. Even if everyone doesn't own a copy they can still play in some limited modes and tracks. Such an awesome game, one of the best Mario Karts to come out in recent years.