I personally think that they are tacky, but maybe that is because the people who put bumper stickers on don't usually stop at one. I saw a car once that was just covered in them and you couldn't even tell what color it was.
No bumper stickers allowed on my vehicles! Not only do they end up looking crappy, they can hide corrosion and allow rot to set in unnoticed. I do have some magnetic bumper 'stickers', though. These are easy to get rid of when they fade.
I personally don't like displaying bumper stickers. I'm a private person and think bumper stickers say things about the people in the car. I like reading them when I'm in traffic, but I always formulate an opinion about the other drivers because of the messages.
I think one or two small stickers on the bumper or on the rear glass are okay, but you see people who have them all over the back and sides of the car. I have an orange T on my back window and a round Pink Floyd sticker on my bumper, and that's it.
I don`t mind having one or two bumper stickers. I do agree that covering your car looks tacky. Like calvin I love to read them while I`m in traffic. I have read some pretty funny ones. Then again I have seen some that are just outright rude.
They will never go on any vehicle that I own, I worry too much about the possible damage of it. My youngest managed to stick one for a place on my office door and that was bad enough, I would hate to see what it does to car paint.