Explain to them the beuty of anonimity and instant transactions, then tell them like any other currency it has its sum of criminals.
The real beauty of bitcoin your really do not need a payment network in the end ( now we need it because so less merchants accept it ), it is safe from any third party (you need to protect your own key though), NOT anonymous ( if you and your address is linked) and anonymous at the same time(just like email address). It make financial innovation a snap (you can organize your own exchange and contracts easily).
And like cash, some criminal use it, but you know what? Cash is really untraceable, bitcoin is actually fully traceable!