My friend Eddie's Uncle Joe heard about this truck driver who narrowly avoided a fiery wreck by plunging his truck through the gaurdrail and down a 100 meter cliff. If he had had his seatbelt on, he would never have been able to jump out at the last second and hang on to the gaurdrail.
But the work-energy principle must be satisfied in every collision, and it dictates that the work done in stopping the driver must be equal to the driver's kinetic energy. The shorter the stopping distance, the greater the impact force. And cases where the seatbelt would not lengthen your stopping distance and decrease your impact force are about as rare as this kind of accident.
Rather than making judgements about safety from anecdotes like the one above, it is wise to consider the evidence from the large database on traffic fatalities.
While the driver with an airbag may experience the same average impact force as the driver with a good seatbelt, the airbag exerts an equal pressure on all points in contact with it according to Pascal's principle. The same force is distributed over a larger area, reducing the maximum pressure on the body.
The presence of an airbag should not be used as justification for not wearing the seatbelt! The seatbelt keeps the driver from moving out of the position where the airbag is effective in capturing the driver and cushioning the impact.