I purchased my Mustang new from the local Ford dealer because it was the color I wanted with the engine / transmission combination I wanted. I wanted to save wear and tear on my truck (since my truck was "old enough to buy tobacco products" (and had over 270K miles on it) and save it for when I need a truck and when I "need" 4WD.
I could have lived without the 17" wheel upgrade; the "Interior Upgrade Package" ; and the (dealership-installed) decals. That's OK. My employer has the "X plan with Ford. It saved me about $1100 - enough to pay for the extended warranty.
Would I do it again?
Only when the next generation Mustang is exponentially better than what I'm driving.