One angle not seen by many. When i bought 5 shares in february 13, they cost me 0.1412btc. When i cashed out in expiry date june 13, i received 0.17724647; 0.17724647/0.1412=1.2549050076. --- Besides the daily interest, my gain in BTC was 25.5%.
Yes, the potential gain depends on how do you measure it (in USD or btc) and the btc price change during the 120 days. Since I entered at btc=800 and got repaid at btc=500 while getting paid interests in usd all the time i made out decently in terms of both, but I would have gained more pulling out btc at their price bottom instead than usd. So bitcoin-trader trading shares are a (risky) way to short btc against usd with a sort of leverage (more gain and risk). An apparently less risky way to do the same -but paying less- is to lend USD on bitfinex (which I am also doing).