Depends on the type of asset you want to create, I'd say. For pure metals your own materials probably would not be much more different as the colors should be (and are) based upon real world values. If you are going for a realistic PBR approach then use the materials. You wouldn't se anyone use a different kind of copper in a movie, either just becasue everybody else uses copper in that place.
How good your model looks is depending on how you blend your materials and how much personality you can give your model. Clever use of masks and such.
You should definitely use your own materials if you want to create a different look or have custom shaders that need different textures. Or just if you feel the existing library doesn't suit your taste.
Otherwise I think it's pretty nonsense to not use something just because you didn't create it yourself. Work smart not hard (or better both).
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For pure metals your own materials probably would not be much more different as the colors should be (and are) based upon real world values. If you are going for a realistic PBR approach then use the materials. You wouldn't se anyone use a different kind of copper in a movie, either just becasue everybody else uses copper in that place.
How good your model looks is depending on how you blend your materials and how much personality you can give your model. Clever use of masks and such.
You should definitely use your own materials if you want to create a different look or have custom shaders that need different textures. Or just if you feel the existing library doesn't suit your taste.
Otherwise I think it's pretty nonsense to not use something just because you didn't create it yourself. Work smart not hard (or better both).