I've been playing more with the voice-command navigation, and it's very good. It got "Parker's Maple Barn, New Hampshire" on the first try, and nailed it. Likewise "Best Buy Nashua New Hampshire" and "Best Buy Concord New Hampshire". (I dropped off four obsolete computers and two monitors for recycling today. Three items per household per day means three items per STORE per household per day, right....? Since they don't record any information about who's dropping them off...)
The GPS receiver is a power-hungry little sucker, though. I bought a USB car charger that fits flush into the 12V socket and effectively turns it into two USB charging ports — one of these, which claims to be able to deliver 1A per port — and it was not able to significantly charge the battery between Nashua and Concord with the GPS enabled. After leaving Concord, though, when I turned the GPS off, it managed to charge the battery from 10% to 70% by the time I got home. This is telling me that the GPS receiver and the Google navigation app together are consuming pretty much the entire output of the charger. (However, the phone didn't actually die between Nashua and Concord, which suggests that if it starts out charged, the charger will be able to keep it charged with GPS navigation running. I totally need a more compact cable, though. This one looks like just the thing.)
I have used, so far, 0.025GB of data in the course of probably three to four hours of total GPS navigation use.