Well, very short term, couldn't you do a test drive (at a car dealer close to your home) of a car with navigation to achieve much the same thing, for zero cost? Pretend you're interested in the car because of the navigation system, and have the car dealer demonstrate the navigation system for you.

Anyway, just as GPSs vary (I tried to help a friend recently set up their new Tom-Tom, after using my Garmin for a few years, and I was pretty lost for a while, the user interface is so different), just as phone-based mapping system vary, so too do navigation systems vary from one car to another. I've head comments at work from people who have car navigation systems, and some like theirs, while other find theirs so unreliable they just use their phone mapping instead.

Also, if car rental companies are notorious for not keeping their portable GPS's maps updated, I doubt they're any better at necessarily keeping the navi systems (on the few cars they have with navi systems) updated.