I have played with voice nav yesterday. It is pretty nice. It took me a while to teach my system to understand what the heck I was saying, but it was worth it. There is a list of commands to choose from, such as "Pan Right" and "Zoom Out". Overall, I was pleasantly surprised. I still need to do more "Testing".
Note: I DO NOT HAVE OEM NAV. Since you did not specify which Voice Nav, maybe you want to know my experience.
I too, would like to know other systems' capabilities.