An interesting theory:
I've read of a theory that states that people with these devices (or pour-in additives) tend to drive a little less aggresively, trying to milk every last mpg out of each gallon, validating the purchase of the device/additive, thereby increasing their efficiency through their change in driving habits alone. This makes the device/additive a placebo, IMO.
If folks would drive sensibly to begin with, they would see the same result without wasting money on these "crutches". JMHO.:)