On my EFI car, I got more MPG after I tuned my ignition and fuel maps.
On our old carburetted cars, we got more MPG after we tweaked the distributor and jets.
It's not enough to do all that and claim that this single part of it is what makes the modification work. You've got to know why and how it worked. If you're simply relying on the gas generated to boost octane and suppress detonation from running much leaner, you could do the exact same thing with water injection (water injection allows you to run tres lean), without spending on the extra current to electrolyze it.
I also find it funny that in the testimonial thread, there was a user stating that the increased MPG also happened when the kit was off. Which indicates the likelihood that it was merely the cleaning properties of the higher octane mix with hydrogen that caused the increase in economy... and not the on-the-fly enrichment itself.
But seriously, no one is debating the added hydrogen helps combustion. That's a given. The question in my mind is whether the extra steps required to release that hydrogen are worth it versus injecting the fluid you're making the hydrogen from (water) straight into the chamber with the appropriate delivery system.