If you're looking for a UK guy getting into this check out the LDV driver (mentioned earlier) who is still trying various designs. They do seem to work and if you build one of the many designs that can produce heaps of hydrogen in a very short space of time, you'll see much better fuel economy.
His baseline fuel consumption was 37.4 mpg and with HHO its 44.7 mpg, and thats using a relatively small home-made unit. Watch:
http://www.watervan.co.uk/videolog.php
I actually sat through all of his clips last night.
