I have really, REALLY tried to stay out of this one, but just can't. Although I didn't own one of the last generation (99-04) Mustangs, I did own two mid 90's era Stangs; a 95 GT and a 96 Cobra. Each was purchased used with low mileage (29k on the GT, 35k on the Cobra). Both were convertibles. I sold each when they had just under 85k on them. I put 100k on that platform, so that's the perspective from which I speak. First of all, both of these cars were reliable and almost 100% trouble free. Neither had a mechanical breakdown (aside from a dead battery on the 95) and each went out the door with 100% of their original components (starter, alternator, fuel pump, clutch, etc.) still intact. I enjoyed owning each of them very much. Had my experiences with them not been so positive, I certainly wouldn't have shelled out the $$ that I did for a new one.
Having said all that, it's my opinion that my 2007 GT Convertible is a much better car. It drives & rides much better, is smoother & more refined in operation, and is faster than either of my previous cars. And for as much as I've read in the 2005+ Talk postings here about the quality of interior materials in the new cars, to me the fit, & finish are much superior to the earlier cars. The plastic door panels in the 95 & 96 creaked constantly. I've experienced none of that with the 07. It just feels like it's put together better.
I loved my 96 Cobra, but truth be told it was a hard car to live with as a daily driver. The ride was rough to the point that it was downright punishing. Last May my wife and I took the Cobra from where we live in NC down to Amelia Island Florida for a week. The ride back, particularly on the stretch of I-95 North through South Carolina (does anyone know what the state of SC is doing with their federal highway funds, because they certainly not using them to maintain the surface of I-95 north), was almost unbearable. And both the GT & the Cobra suffered from front tire wear problems, no matter how many times they were aligned & the tires were rotated. In addition, both cars steered as though the front tires were never quite balanced out 100% right. Each had a slight steering wheel vibration at highway speeds. It was almost as though there was some kind of design flaw in the platform or something.
In the past month that I've owned the 07, I've had many people ask me about it and if I like it. A few have asked me what it cost. I tell them the truth; that it stickered for slightly under $34k and that I paid just just over $31 for it (I got it at invoice). People are genuinely surprised at how little the car costs. And when you think about what you're getting with a GT convertible, particularly if you are able to get a good deal, to me it is a bargain. For less than $32k, you're getting a car with a 300 horsepower V-8, engine, all the necessary options; air, cruise power everything leather, alloy wheels, a stereo with satellite radio & a 6-CD changer, anti theft, side airbags and a cloth top. Compare that to any of the other V-8 convertibles presently on the market and I think you'll come to the conclusion that the GT Convertible is a great value.
Interestingly, the MSRP on my 1996 Cobra Convertible was right at $30k. And that was in 1996 dollars. When compared to that, the new GT is much more car for the money. I agree that the prices for the used GT's do seem high. In fact, it's what pushed me into buying new. My hunch is that the high resale value of the 05's is directly tied to the high prices that those cars commanded when sold new. Perhaps another reason that they are holding their value is because of their popularity, and because they are a good value for the money.