I think the new vs used debate depends entirely on what you're buying. Looking for a newer truck, probably should go new. At least around here trucks hold their value extremely well. Looking for something like a newer Camry or Accord, used is probably the best bet.
I bought a new Focus last year because it was cheap. I got it for $16K and after my trade had to finance $11K. I like it because it's new, has a 5 speed, heated seats and it's a hatch. I drive around 12 thousand miles a year so it should be pretty much maintenance free for the next 7 years. I looked at used cars and it was hard to find anything that had less than 50-60 thousand miles on it which means before long you start looking at timing belts, hoses, brakes, etc. I know I am taking a depreciation hit but if I drive it long enough it really won't matter. It's pretty cheap transportation.
In reply to carbon:
No sweat. FYI, the subject has been beaten to death here a million times, and this probably isn't the best thread to revisit it.
I heard the "This American Life" story too. One caveat is that the dealership would seem to need to be close, but just under their monthly goal for you to get that great end-of-month deal. Maybe the structure of the new car industry makes that situation pretty common, but it was clear from the story that they would not have been giving out such great deals if they had already made their Chrysler bonus goal.
Also, that dealer seemed to be a Jeep dealer, if you go to Truecar and look, it appears to be fairly common to get a new Grand Cherokee for less than invoice. In my local area the invoice on a 4x4 Laredo is $31,759, the "average paid" is $31,292. I don't think I've ever seen that on a car I was genuinely interested in...