I have mine attached behind the steering wheel. In hot weather the glue on the velcro kept getting soft and it would fall over. I got some better stuff but its starting to come up too. On the scanguage I just superglued the velcro in place
After installing it I realized why people kept passing me on the freeway. The analog speedo indicates a much higher speed.
The trip meter is nice for finding the most fuel efficient routes to take. Modifying my route home from work slightly saved about 1/4 liter per day. I like having the LHK and LPH reading displayed. LPH is useful when you aren't moving. Its fun watching the LHK/LPH drop to 0.00 going down a big hill.
You also find out little details about your car. On my 2005 automatic the blue temp light goes off at 59C allowing the transmission to drop into overdrive.