I have visited and investigated, the best I can, websites that offer conversion kits for cars and trucks. The websites swear by this new technology but offers no guarantee it will work on your vehicle. Problem is very few people have the technical knowledge required to install their particular conversion kit.
I have watched videos of people marketing this type of converted vehicle but most are very short and never go into detailed installation instructions.
Alternative fuel systems have been around for a long time. There has always been electric, steam, and even diesel engines were first designed for vegetable oil instead of diesel fuel. The military has had multi-fuel trucks and equipment for years. Solar powered vehicles have been tested as far back as the late 60s. Hydrogen cell vehicles and propane are nothing new. Most Propane gas companies use vehicles converted to burn Propane in their delivery trucks. For these reasons I will not totally dismiss the idea of HHO, but I will remain a skeptic for now.