Anyone who's tried this knows more about it than me, but if you haven't, you should! (This is on a Droid X running 2.1 (604) if it matters.)
I spoke the address and it picked it up entirely without any errors. I don't know if this is simply good voice interpretation or if it does a fuzzy search on what it thinks I said to match it up with a real address. But it worked twice without error.
I shoved my DX in a holster and headed out to my car where I placed it in a dash vent holder. After driving about half a block, I realized I had forgotten something at home so I returned and grabbed it. Back in the car, the DX was still working on a route. I hit the back key to get back to a clean desktop and restarted the app. I was disappointed to see that it had not saved the address in previous searches, but it handled the voice input without error a second time.
I actually had another stop to make first so I headed off in another direction and the Nav program recalculated a route to the original destination.
It only took me a few minutes to figure out that I needed to press the blue arrow on the screen to have the display follow my progress. (Doh!)
I'll have to look into this some more to figure out what other capabilities Google Nav has, but my first impression is pretty good. (I'm really glad I didn't spend the $$$ for VZ Nav when I set up my plan!)
Hmmm... Doesn't seem so good now. I'm trying to enter my home address so it's there when I need it, but I can't get it to pop up the dialog that indicates voice input. I wonder if the server side of this is down.