One of my first games for the GameCube, alongside Super Mario Sunshine. I love this game. I'd go as far to say it's my favorite racing game. Yes. Ahead of Mario Kart and ahead of Jak X.
The vehicles are fun and unique. I spent countless hours trying to unlock them. The Air Ride race courses are beautiful and the music is spectacular as well. I really felt like I was there in the scenery when I would play as a kid. I never cared for Top Ride though. And City Trial is hella fun. I never got to play with other people, but my friend let me borrow his extra controller so that I could unlock the pieces of Hydra and Dragoon for Free Trial, since those required another player to unlock.
One complaint I have is unlocking things, though. Some stars are really hard to unlock, or just features in general. One I always worked on was not touching the sides in Machine Passage. Ever played that one? It's hard to not touch the sides with the sharp turns later in the course.
Overall, it's a great racing game. If you like tripping up your enemies though (at least in the main races), it's not really the game for you. If you're into scenery, music, and working towards unlockables, it's right up your alley.