I personally steer clear of both FAS and FAZ during times of ultra-high volatility.
There is a temptation to try to time the market by betting on one, and then trying to flip into the other at the right moment. It's true that one can get lucky and make a quick buck, but what it comes down to is just exactly that... luck.
The one thing that is sure about these two ETFs is that during periods like we just went through with high volatility, the time decay factor on both funds increases dramatically. So you also have that working against you.