Very Mediocre, definitely try PGR3 first.
Whereas in PGR3, you race with real cars that have incredible animation, sound, and just a sense of pride in being able to master each individual car's handling, in RR6 you get pretty generic cars that all sound the same and have really no special characteristics.
They all drift around corners in the same ridiculous fashion that the Ridge Racer series has become famous, and I believe, infamous for. In PGR3, you can also drift, but the feeling is so much more asthetically pleasing because you actually feel the weight of the car as you're plowing through corners. It's almost overkill to mention that the race tracks in RR6 look awful in this generation of games.
RR6 makes you feel like you're driving Matchbox cars. This game simulates toys, not real driving. You'll be doing yourself a favor buy picking up PGR3 instead.