The stock temp gauge is very slow to react, this is done on purpose, the gauge only ever moves above normal once there is a real problem
I had a Defi water temp gauge and it moved all the time giving very accurate temp reading, these were of course higher in traffic and hard driving, this gauge also had an alarm to warn of high temps, this saved me when the thermostat failed, the stock gauge never moved until the last possible second, it then went from normal to max in one sweep !
It therefore gives greater piece of mind and a better understanding of whats happening
There is a downside to gauges of this quality however, you tend to watch them all the time and worry about every movement, or was that just me !!