There is a mountain of literature as to why leveraged ETFs are horrible long-term plays. In the longer-term you really can't lose by shorting them..
Here is a good article:
http://seekingalpha.com/article/104703-explaining-inverse-and-leveraged-etfs
Here is a good article:
http://seekingalpha.com/article/104703-explaining-inverse-and-leveraged-etfs