I already have one installed in my 2003 Tacoma 4x4, and I went from 17mpg to over 25mpg in the city. It worked so well that I installed a single cell HHO kit in my wife's 2001 Camry and she's getting 35mpg city.
I see a ton of posts of people calling it a hoax who don't even understand how it works. The point is not to run your car completely on water. All it does is produce enough HHO to allow you to run a lean fuel mixture, and that's why it saves gas. Read
this wikipedia article on hydrogen fuel enhancement. This is the concept it is using.
Call it a scam all you want, but I'm enjoying a 50% increase in mileage. See my post here for pictures:
HHO Pictures
