Jeff Jacoby, a conservative columnist for The Boston Globe, has a pretty thorough, well-cited, and convincing assessment of last summer's program, and the news isn't good.
First, it looks like the program didn't increase sales of new cars all that much, if at all. It just concentrated sales in July and August that would have otherwise happened a little earlier or a little later. "Of the 700,000 cars purchased during the clunkers frenzy, the estimated net increase in sales was only 125,000. Each incremental sale thus ended up costing the taxpayers a profligate $24,000."