I have this option and like it very much. I won't go so far as to say that a backup camera is a necessity in the CX-9 but it's pretty damn close. Because of the car's height and the size and shape of the windows in the back, it is impossible to see anything under 3 or 4 ft. tall. This includes other cars when parallel parking, curbs, kids, bicycles, etc. Obviously the nav package gives you a much bigger screen but the rear view mirror image is certainly adequate. If you don't want nav, get the mirror option and you will be glad you did.
I'd also add that the while the nav screen is larger and more of a "premium" option, I've had many people marvel at the camera image in the rear view mirror, including people who have full nav screen backup cameras in other cars. It's definitely a nice solution that provides a degree of safety at reasonable cost.
