After further research, reading, etc. This is bunk.
Here are my 2 reasons:
#1 perpetual motion is impossible (thank the 1st and 2nd laws of thermodynamics)
#2 'increasing the speed of combustion' is not going to result in much, if any, increase in mileage. Especially not what these people claim.
The increase that people are seeing is due to a lean condition created in the engine by 'tricking' the O2 sensor to run lean. It has nothing to do with hydrogen, all the water electrolysis device does is make some bubbles and is used as a marketing tool to make people who do not know the process believe it works.
If you want to get better mileage, just 'trick' your O2 sensor to make the engine run lean and have fun with the repair bills.
Search 'HHO' 'debunking' 'physics' 'engineering' and you'll find some interesting threads/pages from very smart people who will give you the run down as to why this does not work, and will never work. You'll also have some interesting reads from those defending this product without giving any new, or useful, information.