I am looking to put 2007 Mustang GT rims on my 01xlt(2.3 liter RWD)
http://i18.ebayimg.com/07/i/000/9c/9d/da89_1.JPG

According to the rangerstation, they'll fit with spacers.
http://www.therangerstation.com/tech...l_fitment.html

What kind of effect will I see on gas mileage?
I realize the GT wheels are much wider than the stock ranger rims. I realize that the width of the wheel will decrease gas mileage due to wind resistance, but the truck will also be a little lower due to a decreased side wall...so wind resistance will also be decreased. Any idea what kind of gas mileage decrease I will see?

Has anyone put these on their truck? How much do they improve handling, traction, stability? Will they decrease the turning radius? Will they rub(the rangerstation guy says they will fit but I am a little skeptical on whether or not they will rub)?

The difference in wind resistance is so minimal that it doesn't have an effect on fuel mileage. However, the width of the rims increases the rolling resistance due to a wider footprint and will affect fuel mileage, but not that much anyways because you are putting on street/highway tires compared to having an all-terrain stock tires.

Looking at therangerstation link you posted, it appears that the wheels+spacers would be "sunk" into the body as the same as stock wheels; I don't see why it would rub.