I am 25, live in metro Detroit as well, and recently bought a 5.0. I was also concerned about fuel economy because I planned on daily driving it. I understood that it's a performance car, but I didn't want to be getting 12 mpg. I traded in my 1999 Corvette which was not a full daily driver (2005 Civic was). I average about 17 on my commute down M59/Hall Road...heavy traffic. Ride does not suffer anymore because of Michigan potholes. I felt like my Civic was more prone to damage than both my previous Vette or the Mustang.
Can't speak for insurance. Much like your gas mileage, that'll depend on your driving habits. At 25 with no tickets, I'm paying 140 a month on the 5.0.
If you have the financial means to do it, why not? It's true, my Mustang is rolling through traffic 90 percent of the time, but even the sound of the GT500 ABs make me smile and makes my commute home from work that much better. And for the 10 percent of open road...well...
2014 Ingot Silver GT 5.0 w/M6