I guess I'm guilty of Gordon's worst sin. I got my billet knobs at cip1.com. They are threaded, no set screws. I believe they look more "right" than plastic, but to each their own I say. Everything on my car is metal, except the car itself! Steel, aluminum, silver hammertone paint, etc. The aforementioned knobs sit on the white dash with OE Porsche keyswitch with aluminum bezel and aluminum start button. Turn signal knob w/dimmer switch in the end: aluminum and stainless(oh yeah, and handcrafted, by me). Billet steering hub: Carey Hines. Original style e-brake: Jim Dubois(handmade) powdercoat silver. Still need to install my aluminum shifter lever by Carey Hines.
Bottom line, white plastic would look crappy in my white Spyder, but that's just me. Besides, Michelle polishes my aluminum knobs for me. Who would say no to that?