Hi David,

Immeasurably so! In the vast majority of modern engines, if the belt breaks, then damage will be done in the cylinders as the valves hit the pistons. That's hugely expensive to fix. Even if that didn't happen, it would be very dangerous for the belt to snap while you're driving along at speed, as it would mean an instantaneous loss of power, which could very easily cause a serious crash. 

In short, if your belt is scheduled to be replaced, don't take any risks.