Just wanted to chime in since I installed this today.
It's a device called ScanGauge and it shows the water temp, rpm's, hp, voltage. There are more combinations, but that's what I have it set up as.
The temperature definitely changes because of the electric water pump that we have. The temperature ranges from 195*F to 220*F.
I tested it on my buddies 04 G35 and his temperature was pretty consistent at 170*.
I decided that I would rather invest in this device and then pull the car over if I see the temperature rising rather then replace the water pump at 79k when failure could occur anytime even with a new water pump.