I've been beating up on my beloved Mustang quite a bit lately. I've complained about the lack of an IRS and the low horsepower compared to new competitors. So, can I say anything nice about the Mustang? Sure, without being hypocritical, it's still a great bargain performance car.
Even, maybe especially, if you consider the V6 Mustang. There are all sorts of aftermarket parts and kits that transform your "rental" Stang into a real fun and fairly fast car. And even though the GT "only" comes with 320 hp, we all know that with the right choices an owner can pump that well over 400 hp. I would suggest that you avoid the flash and go right for the performance upgrades. How many supercharger or turbocharger kits are there? Too many to count.
In all honesty, if I were to buy a new Mustang, I'd go for the Pony Package and then install an STS Turbo kit or Eaton Supercharger.
If what Igor said in comments recently are true, then things will get a lot better in 2012.
Until then, 05 forward Mustangs will continue to be the bargain performer. Maybe one of these days, when the economy improves I'll buy a used 05-06 Pony and add that STS?