If you want to try Bitmex but don't support the pump&dump club, get a different referral link.
Just mind  high leverage trading is dangerous and many providers (brokers) have a bad or mixed reputation  (not sure about bitmex yet: http://bittrust.org/bitmex)
Often the spread is much higher or less transparent than on simple "spot exchanges" -  even x2.5 Leverage on Poloniex is dangerous enough - and after all you could become a catalysator of Pump&Dump since you never know which direction they may turn to knock you into forced liquidation ( "SHORT squeeze" / "LONG squeeze")
Reasonable/safe Leverage is often considered to be dynamically chosen for each deal between x1.01  up to x3 or so - in slowly moving assets like the DOW jones anything higher limits your chances to change strategy and get out unhurt later than you planed for example  (as of risk and daily fees)
 


http://i.imgur.com/zJhDKZp.png