Does mileage also determine life? I would have 100k in less then 5 years on the car. If I was to trade it in, a car needing a battery would reduce the trade-in or resale value.

Also does anyone know if there is a study on Hybrid driving habits. I wonder if you start to drive more or become less fugal as some one who drives a car getting lower MPG and will reduce driving or plan better on trips to save on filling up.

Hybrids use electric all the time, from what most people claim getting up to speed quickly using gas/electric, then cruising yields awesome fuel economy. The way the Prius works, it has 2 electric motors, 1 spins in reverse working with the larger traction motor to keep the gas engine rpms low, or in the ideal range. Mountains are great too, you may get 30 going up but you won't use a drop of gas coming down. Surprisingly the C takes hills and mountains well (not as good as my mt fit) i mountain bike all the time. I'm talking about large hills too, 4000ft to 8000ft in 10 or so miles.