I have a 2013 Touring with Tech, which has the 5.8" screen and TomTom nav. I like it. The screen definitely doesn't seem small to me. I have always felt it was plenty big as a backup camera. I have also been happy with the nav.
Can I ask what you're considering as competition for the CX-9? Because it really depends what you're comparing it to. I've never owned a nav system except for my cell phone. Compared to iPhone with either Google Maps navigation or the Apple Maps navigation, the TomTom nav is dreadfully slow and the screen is low-res and crappy. Compared to the Toyota Highlander limited that I drove several times, the CX-9 nav is fast and looks great. I also drove a Hyundai Santa Fe with the top nav package, and the screen on the Hyundai is bigger and better than the CX-9's screen.
The process of entering addresses is a pain. The voice commands don't work well so I always use the touch screen. The responsiveness and quickness is better than industry average, definitely better than Highlander, but using that touchscreen after using iPhone and iPad is like going ten years back in time. Soooooo sllloooooooow to register my button presses compared to iPhone.
So, it depends on what you're comparing it to.