Sounds good on paper, but terribly executed. It's short, dull, and just not fun to play.
It's single player is so short,it can be beaten in an hour or two. There are only 5 tracks, each with their own chaos emeralds and coins for characters. So after racing each track about 4 times (give or take), you will have pretty much beaten the game. So the length is the first blow.
Secondly, the level designs are quite poor. Sonic and friends all accelerate really quickly, but don't move all that fast, and can turn tightly with the L and R buttons. They can also cross over water really easily, making it easy to cut corners and obliterate the AI competition. The game also doesn't feel balanced (Amy, Robotnik, and the Eggrobo feel like push overs). Super Sonic is also godly, which I guess is appropriate, but could have been balanced with ring drainage. Multiplayer isn't that great either, but it does rid the issue with terrible AI, and has a tag mode.
And finally, the whole game just feels low tech. The graphics aren't all that great, the music is terribly addicting (the bad kind). And the whole feel of the game is unpolished. Compared to other games coming out at the time like Mario Kart 64, Fzero X, Crash Team Racing, and Gran Turismo, this is just a terrible package that was thrown together at the last minute.
In conclusion, Sonic R, aside from having a few cool ideas, just isn't all that good. You can find a ton of other racers out there that are better for the generation. In fact, any racer at the time was better. You might actually be better off playing Sonic Drift... well okay, maybe not. But Sonic Riders is vastly superior, making this a game you should avoid at all costs, unless you are some hardcore Sonic Fan.