Theoretically, it's bloody brilliant - separate hydrogen and oxygen form water via electrolysis, then burn both in the engine to produce energy and water vapour (no other waste products).
In practice it's a load of old cows*** as you need more energy to seperate the hydrogen and oxygen than you actually gain by burning them.
... and I wrote that lot while slightly (ok, very) pished as well although it took me ages to iron out all the typos and bits that dont make sense