At no point in using the exchange will coins ever leave your control.  someone in slack suggested that blocknet may be the go-to exchange for institutional investors (for obvious reasons), and i agree with that just from a risk point of view.   Also, blocknet has negotated a deal with ethfinex/bitfinex to gain access to their order book, so will spring into life with great liquidity.  right now one can trade on blocknet, but it will become user friendly for mass adoption with release of the UI and multiwallet (hence not requiring full chain downloads).   these are both scheduled for the very near future, we are talking weeks....