I'm building what is a homemade version of that and running it on my civic right now. People that I work with are running the same system I have and seeing pretty decent gains. I expect to see 40+mpg, where I was getting 32-35. google search water for gas. They have books you can buy for ~$100 that show you how to build it yourself. All the parts for what I built can be picked up at a hardware store for the most part. I plan on building and installing them locally for ~200 once I have run it a few weeks and seen the gain myself. Also using one on my dads 03 chevy with a duramax.
Here is a site showing the top 3 HHO setups that are currently out there. The second one (water4gas) is the one I am using. Was directed to it buy people that I work with that are using it with awesome results.
I dont have the book on it, but the guy I work with does. After paying him to make mea coupleof the generators, I see that it can easily be made for a fairly low price.
Well after running it for a few days now, I feel like it at least runs smoother. And I can see that the gas is going farther, but wont have anything accurate until I fill up and go through a full tank (so like the end of next week).