Yeah, when I saw those ads I just did the math in my head... and it wasn't worth it. If gas was $4.50/gallon, then it would save me 1.51/gallon. Assuming I drive an avg of 33mpg (worst case scenario) and drive 12,000 miles/yr:
12000 mi/yr * 1g/33mi = 363.6 gallons/yr
363.6g/yr * $1.51/g = $548.58 savings / yr
...that's assuming gas is $4.50 the entire year, my mpg is roughly 33 and I only drive 12,000 miles in a year. Seems like it would be better for drivers with bad MPG, who drive a lot in a year and during really high gas prices.
(Forget about that Suzuki deal... that's not even worth running the numbers)