Playing it alone? Don't bother. Buy this only if you want some awesome multiplayer.
What makes this game so dang fun enough to get a 10 in gameplay is it's multiplayer. If it was a single player game only it would be a 7 at most. Probably a 6. Anyway, Mario Kart Double Dash!! proves something. It proves that video game meant for multiplayer do not have to be online to be fun. Sure MKDD only gets 4 players, but it's just as fun as about any other online game out there. Single player is short but there are some unlockables to get and the multiplayer never gets tiring.
Graphics are basically average for Gamecube. A lot better than Mario Kart 64, but I'll tell you right now that this game is no Metroid Prime or Starfox Adventures. Some, SOME of the music sucks. Not all but some. It can get annoying and sound kiddy but a lot of the music is pretty good.
Buy this game.
Also, the only new feature besides graphic sand new music is that it has co-op mode too, where you have two people in a kart instead of only one. one player moves it around while the other is in control of items.