Part of what I know about the Prius is this: It's a car. (well, duh.) And that means it's big and heavy (relative to my scrawny transporation system, that is.) Think about it this way: A Prius hauling a single person (most of them do) is getting (let's say) 50 person-miles per gallon of gas, (which contains 31,548 kcal), or 31,548 / 50 = 630 kcal per mile. On a bike, I can cover roughly 15 miles per hour burning 700 kcal, which works out to 700 / 15 = 46.7 kcal per mile. So my system (for a single passenger) is 13.5 times as efficient as a Prius. So if a Prius gets 50 mpg, I get "675 mpg". (I know that's a bit of a red herring.)
In honor of this ratio, I'vc created a graphic. You can download the high-rez version here, if you want to put it on a t-shirt or something.
But the latest data point about hybrids is a truly unexpected one: according to insurance statistics, hybrid drivers drive more, get more tickets, and have more accidents than non-hybrid drivers. (Perhaps they are distracted by their hypermiling techniques?) According to the study cited here, the decreased guilt associated with improved fuel-efficiency increases the number of discretionary trips by up to 25%, largely offsetting any petroleum savings.
It is, after all, just a car.