Parents: Sega GT is completely safe for the kiddies.
Probably the best advice I can give is this: Sega GT is essentially Gran Turismo DC. If you loved Gran Turismo and Gran Turismo 2, you'll love Sega GT.
If you couldn't stand those two games, then steer clear of Sega GT.
If you missed out on the Gran Turismo phenomenon, then Sega GT is a great way to introduce yourself to the genre. The game has the same appreciation for sports cars, the same understanding of the physics of racing, and the same challenging gameplay—only this one looks a bit prettier on Dreamcast. It's definitely not a must-have game, though.
If you don't have such specialized tastes, there are plenty of other Dreamcast driving games that are more suitable for a wider audience.