Anyone use it and how did it work for them? For that matter WHAT IS IT and how does it work?

Water wetter is a surfactant that decreases surface tension of water. It can improve heat transfer. But it can also destroy the chemistry of your coolant's anti-corrosion package. You have two risks: the virtually non-existent risk of overheating in a properly maintained cooling system, or the omnipresent risk of internal corrosion within the system. I would rather protect against the latter and avoid WaterWetter.

Enjoy!
http://www.redlineoil.com/products_coolant.asp?subCategoryID=4&coolantFlash=1
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93' 240 Classic Wagon 220K & 92' 740 Wagon Regina/Rex 90K

Water Wetter is designed for track use. Conventional coolant already contains anti-surfectants. The problem is, it is also extremely slippery, and this is dangerous on a racetrack. Many roadracing organizations ban coolant for that reason. The problem is water alone often can't cope due to the microscopic bubbles reducing surface contact. Water Wetter reduces this and increases the surface contact of the water to the engine and radiator without the hazard caused by spilled coolant on the racetrack.
The long and short of it is, if you use a coolant/water mix, you don't need it. If you race in an ordanization that bans coolant, then you do.
Cheers!

Seems to me the thermostat and radiator do a good job regulating the temperature and water wetter isn't needed unless you have over heating problems. I know when the thermostat on my turbo was broken in the open postition the engine was cooler than it was supposed to be, so there is plenty of cooling capaciy in your brick.

Water wetter should just affect the position of the thermostat. Temps should be the same- again unless you have an overheating problem.
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Paul NW Indiana '89 740 Turbo 128,000

I have used redline water wetter in my mini, with a water/coolant mix, and found that the temp gauge sat slightly lower with this additive than without. I have not taken actual before/after temperature readings, but it does seem to help the cooling system work more efficiently. I had no problems after adding it, and for the minimal cost ($15.00 CAD) I have added it to my 940 as well. It should help heat transfer, which should make for better heater function in the winter as well, and just thinking about that makes me warmer ;-)

Idonknow minidave?? Seems if the temp gauge was reading "slightly lower" it would reduce the heater output but ..... maybe it's offset by better heat transfer or maybe it's just the way the surfactant interacts with the temp sensor or.... Bottom line, if you feel warmer it's working better!!

Water Wetter is an additive to increase the heat transfer by reducing the surface tension of the water. When mixed with water (no coolant), it does help the engine run cooler. Hopefully someone else can offer some experience running it with a coolant mixture.

Seems like that is more likely to encourage leaking, especially at pressure. The car is designed to manage the heat automatically, with regular maintenance. Why should water be better - why would you want your car to run cooler - possibly even outside its most efficient temperature range???
You would be better off spending the money on repairs to your cooling system.
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1990 740 Turbo, on its way to stock specs, maybe beyond

Uberoptiker's post above reflects my experiences with Water Wetter. If you can't use coolant, it can help quite a bit. If you're banned from using any additive to the cooling system, it doesn't change the color of the water enough to be noticible.