Dear Mythbusters,
is it true that an automatic car uses more gas than an manual car? And that a manual car has more control over an automatic one??

An automatic diverts some engine power into gearbox control. Manual gearboxes don't. For the same spec car, the auto will always be thirstier than a PROPERLY DRIVEN manual.
To figure the control question, have a look at the cars driven by people who really need control. Race car drivers. You can count the automatic racers on the fingers of one foot.

its important to remember that fuel economy is mostly down to HOW you drive, rather than WHAT.

I would say that your myth is plausible. While the driver of a manual car has the ability to change gears according to the conditions (and drive it in a fuel saving way), the automatic car changes gears whenever it "feels" to do so. The automatic car does not take into account (yet) that there is a hill, stop sign, another vehicle, etc. ahead, so it may use fuel/gears unnecessary.