I'm not a fan of car bumper stickers, magnets or stickers. Thoughts:
College - These are OK I guess.
Sports Teams - I can kind of understand these but only use one.
Honor Student - Retarded. Even more retarded are the ones that say, "Blah Blah school honors ALL of their students."
Beach Destination (ie: OC for Ocean City) - These are OK if you actually own a home there or go there almost every weekend.
Cartoons showing how many family members you have - Stupid. What happens if someone dies? Do you take one off? Maybe I should sell halo and wing stickers that can be used in this case.
Political - I really hate these. And if your candidate loses take if off for God's sake.
Magnets saying which breed of do you love - I hate anyone that has one of these. Why do you have to advertise you have a Yorkie?
OK that's about it.