I asked David Schwartz about his thoughts on Bitst
Post# of 997
Bitstamp has been reliable for many years and I would feel comfortable storing moderate amounts of XRP there. But for large amounts, I recommend using something like a nano s.
The risk is primarily that regulators will push on bitstamp and threaten their banking relations or freeze their accounts. Also, there's always the risk of a hack or theft.
Bitstamp is one of the exchanges I trust the most. I know their founders, and they've been supporters of Ripple since the earliest days. But you never know what can happen.