Doesn't anyone read the label, much less go to the mfg web site for more info? Assuming Redline Water Wetter, the bottle states it is primarily for use in systems that don't have antifreeze. If you read the charts, the best effect is obtained in systems that don't have antifreeze. It only provides a small overall effect in systems with antifreeze.
That said, I have used it because I felt it might help reduce the local temperature at some of the hotspots near the combustion chamber.
All the previous comments about not seeing much change due to low resolution factory gauges and the thermostat acting to maintain a even overall temperature are accurate, and just plain common sense. Think about it.