I happened to get one of these devices for Christmas a while ago. The device plugs into the diagnostic port under the dash and does useful things such as tell your speed, gas mileage, temperature, and RPMs. In addition, it can tell you what mechanics charge copious quantities for. It has a nifty feature that can let you see what OBD codes cause your check engine light to annoy you.
The primary reason I so strongly desired one was my VW Beetle's minimalistic dashboard. Essentially, it had only four informative displays. Speedometer, Tachometer, fuel, and a yellow Problem Light in the form of a check engine lamp. In northeast Ohio, winter is cold. I mean cold. Turning on the heater only to receive a blast of cold air is rather discouraging. Knowing that the engine is at only 10 Celsius gives me fair warning. And as it turns out, the analogue speedometer display is under by about 5 mph which might lead to accidental speeding.