I'm giving it a try. I don't have any results to report, but from the website it appears that it significantly increases the boiling point of the coolant thereby reducing boil off. It also reduces the viscosity to allow better flow of the fluid throughout the engine (similar to Water Wetter). According to Evans, it doesn't reduce the engine temperature.
Water Wetter is a supplement to the coolant currently used in most cars (ethylene glycol). NPG+ (propylene glycol) is a complete coolant replacement with much improved properties.
I understand that Sean Roe has experience with this stuff in racing applications.
NPG is great stuff for a race car, but I don't consider it practical for a street car. It can't be mixed with any water, so your entire cooling system, including engine, must be completely evacuated of water.
Also, what happens when you blow a radiator hose out in the middle of nowhere? Billy Bob's Pork Rind Emporium and Auto Parts ain't gonna have any NPG.
Also, we stopped using Water Wetter a few years ago after discovering Royal Purple Super Coolant Additive. In our street and race cars, we could see an 8-10 degree decrease on a real coolant temperature gauge, whereas we saw virtually nothing with Water Wetter. Royal Purple additive really works! (And of course, I carry it in stock...)