Battery Cars

I have been looking at the new Chevy Volt,  slated for initial release Fall/Winter 2010.  This vehicle runs on total battery technology for the first 40 miles.  Once it reaches this point a small gas engine turns on to recharge the battery.  This is brand new state of the art technology and one wonders how many cars could be replaced with these new vehicles and stop the consumption of gasoline.  Plus with the Federal Government giving a $7500 tax credit, and various states also doing the same, why would you not opt for a battery car?