I just received an SLS part from an engineering firm that is glass-filled nylon and I'm impressed: stiff, even finish, very functional, dye-able. We also use Shapeways for 1-4 parts but you don't have quite the materials I can access at more traditional rapid-proto services. So...offer the GF nylon and you get more of our prototyping quantities. And cheap/functional/detailed parts are always a good combo.