usually the belt snaps because it was either overtightened or because it was installed improperly. I've had 2 belts break on me in the past 2 years, both broke right after I installed or reinstalled them. Overall, I've had great luck with the NAPA generic belts.
The engine still runs fine, you just don't have the power of the SDS, AC, or alternator. As long as you have a battery with some juice in it, you'll be fine. And its all fixed by replacing the belt which is about a 30 minute task.
Another cause of belt failure is making sure the alignment of the SDS pulley is perfectly in line with the other pulleys. It good easily jump a notch and start self destructing itself. (Personal experience)