quoting AKSAR: "Having some means to cut seat belts makes sense for rescuing others. However, I don't think there is often much need for a tool in self rescue. In the vast majority of cases, the best self rescue tool is going to be your hand, simply reaching the buckle and releasing it in the usual manner. "
War story time: Many years ago, I was riding in the passenger seat of a cargo van when it did a 3/4 roll. I wound up hanging on my side from the seat belt (at that moment I really became a believer in seat belt use).. Unbuckling was automatic and easily accomplished.
I would sy my mental faculties were clouded. My first thought was to take pictures for the insurance company.
Another story showing that buckles can jam closed. A friend of mine ended up shiny side down while off roading in his jeep, and was hanging upside down from the belt. His body weight was enough to jam the buckle. Once he got his weight off the belt, he was able to unbuckle himself.