Mustang has been to Maine and upstate New York to Watkins Glen International.
Photographic evidence. May 2009 in my 05.
I've rented cars in Alabama, Florida, North Carolina and South Carolina.
All tolled, I've driven these above states plus, Washington, Oregon, California, Nevada, Arizona, New Mexico, Texas, Oklahoma, Arkansas, Tennessee, Virginia, Pennsylvania, Vermont and New Hampshire.
Lots of nice places. Most noteworthy would be Florida keys, Oregon coast and PCH in northern California, as well as Blue Ridge Parkway in Appalachians.
Honorable mentions to twisty secondary highways in upstate New York (Finger lakes, Lake Placid areas), Vermont (Lake George to Canadian border via Stowe) and New Hampshire (Mount Washington-North Conway and the "Kanc").