I've fumbled around with the portable NAV units in several cars and I'm really glad to have the NAV integrated into my TCH now!
I really like my TOMTOM devices but I was always worried about them being stolen or falling off the windshield. Wires hanging all over the place was also a real pain.
The integration is worth the few bucks of depreciation to me.

The portable NAV units do have their place however. I still use mine when rent a car on trips.