Used cooking oil from restaurants and such works fine - but only in very limited circumstances. It's okay for a relatively few crackpots and curmudgeons, but there isn't enough around to make a dent in our energy dependence. More power to 'em, but now that the "secret" has gone mainstream I predict they will start charging $$$ for this.
There is more than enough oil shale and coal (gassification) to end our dependence on foreign oil and to last us for longer than we need. When conventional oil reaches and maintains a certain cost, these will become viable.
Every little bit helps.
Plus, if we can start using more corn to make bio-diesel maybe we can loosen some of the tariffs on sugar and replace that horrible high-fructose corn syrup as the nations sweetener of choice.