I made the decision based on the bling factor. I didn't really NEED the Navi, I just WANTED it!
Yeah, if you've got the extra cash to fork out for it, get it. It's really nice. Plus, it's great to have it already installed in the car. You asked a question about the Honda Navi being tied into the IMA. Yes, kind of. It displays a little more information: like current MPG, distance to empty, average speed, etc.
But then again, everyone else makes a good point about portability. Having an aftermarket unit makes it possible to take it with you in any car you may drive. Like, going on vacation to Delaware. "Yeah, we're in Delaware..." You fly there, rent a car, pop out your handy handheld GPS, and you're good to go!
I have both. My HAH has Navi. And I also own a portable GPS. I mostly use the portable for my motorcycle, but take it with me on trips everywhere I go.