Freaking addicting.
Special Stage is an exact console transfer of Initial D: Arcade Stage Version 2. The gameplay remains exactly the same, and it even better if you have a driving wheel. The graphics, for some reason, are slightly worse than the arcade game; with the framerate sometimes dropping for a fraction of a second, and parts of the cars looking a little pixelated. However, you don't notice these things much when you're driving.
The addition of a story mode that was not in the arcade game is one of Special Stage's best features. It allows you to play through the manga, first as Takumi and the eight-six, then as the Red Suns, and finally as Project D.
The arcade mode, both versus and time attack, is very well done. The versus mode, no matter how many times you play it, really never gets old. Same with the Story Mode. You never really tire of going back, trying to get a better time or finally beat Keisuke.
Also adding to the replayability is the sheer amount of cars you can use for the arcade modes. 32 cars with anywhere from 3 to 5 or 6 tune courses for each one, for a grand total of around 130 different cars!
I have to mention the soundtrack, too. As an Initial D obsessee, I nearly passed out at the soundtrack list. 35 different background tracks from 1st, 2nd, and 3rd Stages, with songs such as Speedy Speed Boy, No One Sleep in Tokyo, and Running in the '90s. Let me tell you, there are few things as fun as driving Akina with your favorite BGM going.
To sum up, if you're an Initial D fan, you owe it to yourself; buy a Magic Slide and import this. If you're not so big on Initial D, get it anyway. Special Stage will still give you a truly phenominal racing game.