SCAM !!!! I read extensively about it and the science is ALL WRONG. I studied chemistry in school so I know.

Even if you don’t understand the science behind it to know, ask yourself this one simple question: if these devices worked, why don’t the car companies install them at the factory and sell more cars?

Because they don’t work…

Good Luck…

P.S. If you go to these HHO sites, they will tell you that the HHO become a catalyst for the reaction inside the engine. NONSENSE ! In chemistry Catalyst is does NOT react at all and just help other reaction. Hydrogen and Oxygen in piston would be immediately consumed (and so by definition cannot be a catalyst).
References :

First, there is no such thing as HHO, this is an invention of many scammers such as Brown, Klein and others who simply sought to fleece investors that might believe their psuedo-science.

When you perform electrolysis of water, you end up with a simple mixture of hydrogen and oxygen. The chemical formula for that is 2(H20) becomes 2(H2) + O2

There’s nothing special or magical about this mix of gases.

However, to answer your question – yes you *can* run an engine on this mixture of gases and on first YT video I link to below you’ll find a YT video of a lawnmower running on this gas.

But, don’t be fooled into thinking that you can simply stuff a jam-jar with some baking soda under the hood of your car and either improve your fuel mileage or get rid of gasoline altogether.

Just look at the video in the first link I’ve provided below — do you see the size of the electrolysis cell needed to create *just* enough gas to make that lawnmower run at a low idle? And look how much electrical energy is involved – 1,700 watts.

If your car has an 70A alternator, the most electrical power it can deliver is 70×14 = 980 watts. So you couldn’t even generate enough gas to run that lawnmower. How on earth could you make enough to run a 4, 6 or 8-cylinder auto-engine?

Unfortunately, some scammers have spotted an opportunity to use this bad-science as a way of defrauding thousands of people out of their hard-earned cash by coming up with info-packs, eBooks and kits that claim they’ll double your mileage or even let you run your car entirely on water.

These scammers are defrauding a gullible public of millions of dollars right now — it’s the biggest scam on the internet.

It’s impossible, it doesn’t work and it can’t work.

No doubt there’ll be people who answer your question by saying "yes it does work, I’ve got an HHO system in my car and I’m saving…." Well watch the video in that second link to find out why these people claim it works when science says that it can’t.
References :
http://nz.youtube.com/watch?v=lSrYX8Va2Ew
http://nz.youtube.com/watch?v=LSnSxAMjXq8
http://aardvark.co.nz/hho.shtml