There's a bunch of commentary on the program out there, and Brad Plumer's post gives a pretty good roundup. If this statistic from ClimateProgress is right, it's good enough for me to consider the program a success: the better gas mileage of the new vehicles will save 72 million gallons of gas per year. At $3 per gallon, which I imagine is a pretty low estimate for gas prices in the coming half-decade, the program pays for its $1 billion cost in 5 years. On top of that, you get some stimulus benefits, and the environmental benefits of using less gas.
Plus, people get nice new cars. Not that I'm a big car guy -- I've never owned one, and hope to avoid the need for car ownership as long as possible -- but apparently people enjoy having new ones and we utilitarians are happy to see people get what they want.