I've heard their stuff is decent. I'm more familiar with Steeda, Lakewood, Maximum Motorsports and BMR though, but UPR is pretty popular. And their prices seem to be pretty good.
Good read on the Fox mustangs from a couple years ago. http://www.motorauthority.com/news/1042772_fox-body-mustang-makes-cnn-moneys-top-ten-investment-cars-under-5k I myself have definitely noticed a jump in the prices already. So another 3-5 years and the value just may sky rocket like it has...