Yes a 'semi-scam' I would guess. More mis-leading than outright scam. They probably fit an electrolysis system to your car run from the alternator and feed the hydroxy gas generated directly into the air intake.
There are some claims that this improves the burn efficiency of the fuel and therefore reduces fuel consumption. There are plenty of details out there on how to build these.
I don't know if this is what they have or not but I do know:
A) It is not a "fuel cell" and calling an electrolysis device a fuel cell is misleading at best. (they are of course opposites) (and making comparisons to Honda's new fuel cell car is not credible)
B) It does not cost $3500 to make one of these devices there are plenty of instructions on the web and it can be made for under $200.
C) It is unproven that it adds any fuel efficiency boost, let only 40%.
I notice they are now selling these on trademe.co.nz as well!
http://www.trademe.co.nz/Browse/Listing ... ermanent=0
Buyer beware I would say...