Well the argument that you cannot create more energy than what you put in is fallacious.
Don't get me wrong, the laws of thermodynamics are not fallacious, just the argument of using them against HHO.
For an internal combustion engine, HHO is an additive, not a fuel. The small amount of HHO injected should not be seen as pure energy source that moves the car.
Instead, what HHO does is improve some properties of the actual fuel, gasoline/diesel and other forms of fossil fuel.
One of the most important properties that is changed by HHO is the flame speed of the gasoline. That, in its turn, has many positive consequences on the gasoline burning efficiency.
I'm not going to go into details, I'll let you research by yourself.
This has been studied by many, including NASA.
Here's a link of the study conducted by NASA in the 70s.
http://ntrs.nasa.gov/archive/nasa/ca...1977016170.pdf