I own a Prius. Fuel efficiency doesn't seem to be any more variable than the conventional cars I've driven. The distance remaining estimate seems reasonably accurate in that it goes down ~50 miles after I've driven 50 miles.
Whenever I drive a Prius with ZipCar, the MPG numbers change wildly while I drive, and even the average MPG can change by 5–10 miles over the course of a decent drive. I assume the distance remaining estimate is based on that average MPG number, but since I have seen it change by significant amounts, that means the distance remaining estimate certainly is reasonably inaccurate.
If you always drive your car in the same manner then sure, you'll probably have a reasonable estimate. But similarly if all you do with your computer is browse the web, you'll also have a predictable battery remaining estimate. But while many people do drive fairly consistently, your usage of your computer probably has a lot more variety in terms of energy impact.
