I personally dont have any on my car, simply because I dont have anything worth advertising about besides my stereo, and im not going to put "Steal Me" on the back of my car..
I dunno i like the way they look too but i wouldn't over do it... Things like "AEM" and "GReddy" arent bad, because they could mean anything "GReddy" could be your exhaust, or it could be your Turbo, people dont know !
Whatever works for you..
Have fun though =)