I'm neutral on most bumper stickers. I think they fall into three categories, brag/statement, identification, and humor.

Locally, motorists have adopted those oval international country identification disks, you know the ones that not so subtlely remind us, "I went to Sweden and bought this expensive Volvo and you didn't!", to indicate favorite state or even ISPs, so we see little green ones with VT or MV on them (MV.COM is a local internet provider originally inhabited mostly by computer gurus.)

Tom and Ray Magliozzi of "Car Talk" started a traffic safety program against talking on car phones while driving, with a bumper sticker that says, "Hang Up and Drive!"

Many medical providers have a small cadeuces with the notation MD or EMT indicating their ability to render emergency medical service. My last two cars had small stickers with vertical green, red, and yellow strips, indicating to fellow veterans that I had served in Viet Nam, and to others that I had no sense of color matching.

When I was in England, semi-trailers/articulated lorries wore a "Long Vehicle" tag on the back to warn depth perceptionaly challenged people, and many motorcyclists adopted a smaller similar tag that read, "Short Vehicle". I may still have one. And I do enjoy the occasional thoughtful humorous nudge like, "Nuke the Unborn Gay Baby Whales", or the Driving Training Instructor whose license plates read, "IPRAY2"