All of the turn-by-turn data that is displayed by the HUD is supplied by a smartphone running either the Garmin StreetPilot app for iOS devices or a Navigon navigation app for Android, iOS, or Windows Phone devices. I did my testing with Navigon USA & CAN on a Google Nexus 4.
After you get the HUD installed on your dashboard, select either the film or the lens to view it with, and power the device with the power cable, it will enter a pairing mode in which it will illuminate and animate its display while waiting for a host phone running a compatible app to tell it what to do.
On my smartphone, I simply activated Bluetooth, opened the Navigon app, and selected the HUD from a menu. All of the pairing from that point is automatic.
Then I selected a destination in the app like normal and, alongside the onscreen and spoken turn-by-turn directions supplied by the app, the HUD displayed navigation and GPS information.
The HUD's seemingly holographic display shows a large arrow that indicates the direction of the upcoming turn indicated by the software and a large number indicating the distance in miles or feet to that turn.
Below that, you'll find a lane guidance indicator. When approaching a highway exit or large intersection, a number of small arrows will appear at the bottom of the HUD, corresponding to the number of lanes on the current road. Arrows that are filled indicate lanes that are valid for the current route. Arrows that are hollow should be avoided; it's pretty simple.
Also displayed by the HUD are the estimated time of arrival, your current speed, and the speed limit for the road. If traffic is indicated on your route by the host app, a small red traffic icon will illuminate. Likewise, if you approach a red light or traffic camera.
The host app can even download firmware updates using your data connection and push the updates to the HUD via the Bluetooth connection.
When using the HUD, you can also mount the host phone on the dashboard for occasional viewing. The HUD lacks a speaker, but spoken turn-by-turn directions will continue to play, either through your phone's speaker or a car stereo connected to the device via an auxiliary audio connection or wirelessly through Bluetooth.
Floating above the dashboard, the HUD's display sits almost directly in the driver's line of sight, but because of its transparent nature, it doesn't obscure vision at all. The readout is bright enough to be viewed in broad daylight, and an ambient light sensor allows the HUD to automatically dim at night.
The Garmin HUD retails for $149.99 (although you can already find it on sale), but the display also requires a Garmin app on your smartphone to function, so go ahead and factor an additional $50 into that price if you're not already a StreetPilot or Navigon user, bringing the cost of entry to about $200. A $50 app and a $25 dashboard mount would get the job done just as well, for a lot less money.
Then again, the Garmin HUD won't display pop-up notifications, doesn't have a touch screen to tempt you with interactions, and allows you to keep your eyes on the road even when a little lost. Also, it just looks cool. At its asking price, the HUD is more of a cool gadget than a proper safety feature, but any increase in safety is a good thing.