The resale value on these cars is anything but horrible. My experience is only with the GT convertible, but the reason I ended up ordering a new 07 is because the low mileage used 05-06 cars were nearly as expensive as a new one.
As a general rule, I've found that V-8 Mustangs hold their resale value quite well, particularly if they are purchased used with low miles. I purchased a 1995 GT Conv w/29,000 miles in March 2000 for $13,500, and sold it in November 2002 with 86,000 miles (lots of Lejeune to Quantico miles) for $10,500. Next I purchased a 1996 Cobra Convertible w/34,000 miles in October 2002 for $16,400 and sold it in Sept 2006 at 82,000 miles for $12,000. If you look at those two examples from a standpoint of number of months owned and the difference between purchase price and selling price, they each cost me roughly $100 per month to own ($110 for the GT, $94 for the Cobra) in terms of depriciation. Viewed another way, I paid $0.091 per mile in depreciation on the Cobra and $0.052 per mile on the GT. Not bad for cars that are as much fun as V-8 Mustang Convertibles...