I have them on my 986 S with the GT3 rims, and it does look better with them. I bought the car second hand, and I did not add them myself...but I was always wondering why my car looked a bit better - seriously, I was wondering whether I had the 'my kid look good syndrom' :-) I parked it next to a friend, also having a 986 S with 18" too (Carrera's) but WITHOUT the spacers. And it DOES make a difference...
However, not sure I'd order it on a new car