I have my logo on my back window. It isnt large but its clearly visible. I haven't gotten a call or even a comment on it, and I have had it for a few months.
I have something a little bigger than Greg's on my tinted rsx. 5% tint with silver lettering. I actually got my first call from it saturday from somebody driving behind me. So over the course of 4 months, I think only one person called.