The Octopus for Tourists app only lets you top up the mobile Octopus in non-HKD currencies and definitely has a slight currency conversion markup. Last summer, I used it regardless and found it to be quite convenient for MTR travel, and I didn't have to deal with any cash exchanges or ATM withdrawals (which often incur their own fees unless you have something like the US-based Schwab debit card). On iPhone, you can set the Octopus to Express Transit mode so you can place your phone down without unlocking it at the transit gates - far more convenient than taking cards out of wallets.

Still, though, with the launch of contactless Visa cards at the MTR gates, I bet I will refrain from getting another mobile tourist Octopus (I refunded my last one) and just use my contactless Visa CC this upcoming summer.