As a disclaimer, let me state that the average age of the cars my family drives daily is about 12 years, so maybe I'm not the most normal person to talk to.
Heck, when you add in project cars and work vehicles, the average age is more like 23 plus years.
The rule of thumb always been that a new car loses 70% of its value in the first four years. I suspect that the poor economy has driven used car prices up and forced new car dealers to cut their prices. But I still suspect a four year old car is still going to run about 50% or less of the value of new.
With new cars running around $24,000 and up, that's $12,000 of lost value, or $3,000 a year...in addition to gas, interest on the car loan, insurance, maintainence and repairs.
Maybe some folks won't miss $3K a year. I and most others would.
As far as when I would buy new?
1) If you are in a business, and a tax credit is available for new equipment and not for used.
2) If you have enough money that it doesn't matter.
That's really about the only places it makes economic sense to buy new. Some might think someone with ZERO mechanical ability/sense should buy new to save on repairs. If that's the case, buy gently used and get an extended warranty. I doubt that such a warranty would be equal to 50-70% of the car's cost.