No one can really tell you when or how long a battery will last. Yes it might last into the 2-3 hundred thousand miles, but as we all know S%^T happens. I know I can't take a $ 2000-3000 hit.
I suspect most of us are more worried about the price of gas and saving money than we are about the green of these cars. I try to be as green as I can but am still worried about the chance that I will have to all of a sudden have to replace one of these.
I have decided to buy another Mercedes Diesel. My brother regularly gets over 40 mpg in his Blue-Tec if he keeps his foot out of the over 400 Ft Lbs of torque that car has. And no battery to worry about.