We installed a Seibon carbon fiber hood on the Mach V STI. The pictures here show it in its raw carbon form (it's finished in a clear gel coat), but it'll eventually be white like the rest of the car. We'll probably leave the side vents in carbon just to show the underlying material.
The hood fits pretty well -- it's molded underneath just like the factory hood, and it's got holes for the factory-issue windshield washer nozzles. It does come with silver aluminum mesh inserts, which we thought didn't really look good, so we removed those. The factory hood scoop doesn't have any mesh in it either...
The original purpose of carbon hoods on cars was to save weight. In recent years, a lot of carbon hoods don't actually reduce weight at all. The use of lightweight aluminum on the 02-07 WRX and STI, for example, meant that a carbon replacement weighed the same as the metal hood. But in recent years the price of aluminum has soared, and Subaru went with a steel hood for the STI. At 22 pounds, the Seibon carbon hood weighs a lot less than the 45-pound stock one. Hooray for actual weight reduction!
Note that we don't have hood pins on the car, but I've always recommended people use them with any aftermarket hood. Still, the Seibon hood is made with a metal plate imbedded in the fiberglass that holds the hood catch. Given the minimal amount of use our car gets, I may hold off on the hood pins until some future time when we're headed for the race track with the car.