The CR shares bus service with GF and Poly. I recall that CR was first in drop off and pick up (meaning you'll stop at GF and Poly when you leave but not when you return). If that is backwards, I apologize.
I've both had a car at the Poly and driven to AK and DHS. I think both were a bit faster by car on the average and the car was more comfortable for me. I think a car would save about 5-15 minutes each way, depending on the luck of just exactly when the bus arrived at the stop. FYI you get free parking at the parks.
Travelling by car is _much_ faster to Downtown Disney, and the water parks. You could easily save 30 minutes each way. Also, if you book an character meal at DHS or AK which seats before park opening, a car will save stress and time, as the buses only run to the character meals at 30 min interval (if at all).