The old earthmate doesn't do WAAS and doesn't have as many channels, so it takes a lot longer to acquire a lock. The USB Earthmate works pretty well, but I'd rather have a Garmin GPS 16. The HVS version is suitable for powering directly from the car's electrical system, although you'd probably want to wire up a relay to the car's accessory line to turn the power off to the device when the ignition is off.
It's quite a capable unit, although it's twice the price of a USB Earthmate or one of it's competitors. There's also the GPS 18, which is somewhay cheaper and smaller (but not weatherproof for external mounting) that comes in both USB (powered by the USB port) and serial (powered by 10-14v from your car). It comes with Mapsource North America or Mapsource Europe, depending on where you buy it.