'Utility' was my #1 reason: Can the car carry a family of four (one infant,) a golden retriever, and a healthy amount of 'gear'? The Prius (with its hatchback) is a winner here. (Over a Civic Hybrid.)
Safety was #2: Is it safer than the average new car? Yes. With additional airbags, and stability control, it's a very safe car.
Environment was #3: We seriously considered a Jetta TDI wagon to fuel with biodiesel, but the difficulty of buying biodiesel was beat by the simplicity of plain old gas in a hybrid. More than enough to balance out the extra 'utility' of carrying capacity in the Jetta wagon. (We may end up getting a used TDI wagon to replace both our Ford Explorer and the small 'commuter car' Hyundai Accent that my wife is driving.)
Financial incentives were #4: This includes mileage, and the fact that the more expensive Prius' insurance is about 50% that of the $10,000 cheaper Hyundai Elantra GT that was also considered a 'finalist'. (The Insight was ruled out at 'utility'.)
Political statement was #5: The Jetta TDI on biodiesel would have been a greater political statement, but there was some consideration. I contemplated a custom license plate of 'NO OPEC', but decided against it because I drive the Prius to appointments, and probably shouldn't piss any customers off.
'Geek-Factor' was #6: If the dealer had been willing to compromise by just $200 more, we would have gotten the top-end package with navigation and Bluetooth; but even the 2004 package 7 we got has plenty of gadgetry for me.
As for other vehicles? Well, if we ever win the lottery, my wife wants a Hummer. I'd insist that she get an H1, and convert it to run on
greasel, making it probably the most environmentally friendly Hummer on the planet. (Double-checking, it appears that the linked-to company has already converted at least one Hummer...)
And if I won the lottery, I'd also convert a Enzo Ferrari to an electric vehicle, a la the tzero