here in Massachusetts you have to wear one, if you don't they will pull you over and cite you, I believe we are one of the first states to have such an aggressive policy, it used to be that they could only cite you if you were stopped for another offence however as of a few days ago the police were given the ability to stop for only seatbelt violations.
Personally if I am on the highway I generally wear one but if just making a quick trip I tend to not...I find them to be somewhat annoying as I have yet to find a seatbelt that doesn't cut into my neck no matter what the adjustment and also I hate it when they lock you in place from time to time making it so I cannot lean forward.