I got the Nav system and am very glad I did!
I used to use a TomTom device (in other cars) mounted to my dash or stuck to my windshield with wires hanging everywhere. The TomTom is great...any nav device is great!
When I got the TCH with Nav, it took me a while to get used to the Nav system and how it operated. I've grown to love it. Even more so after I went on a trip where I used a rental car with my TomTom. The TomTom device did not have all the features I had grown to love in the TCH Nav. The big diff are the screen sizes. The larger screen size in the TCH enables more options on the screen (ie dual map display).

XM Radio display is great
FM Radio and CD/mp3 text is cool too but some of the display is disabled when in motion.
Cell phone integration and phone book are great. My TomTom integrates with my phone too but not nearly as well.

Do you really want to by a new car then stick a suction cupped device to the windshield and have the wire/s runing across the dash?

I feel that the Nav system is a must have!