I've had a couple of issues. The first has happened twice, the screen stays blank but the voice commands work. The second issue is more frequent, it refuses to locate me. I let it go on our trip to Minnesota and it searched for over two hours before I restarted the phone to get it to work.
I have had issues with the Google Nav app as well. One time it had me get off the freeway, go two miles East and then take a tollway 2 miles West right back to the exact same intersection.