BTCJam quotes a 10% default rate resulting in 19% annual returns, which they also claim is a low default rate for peer-to-peer lending.
I can tell you that in my experience, thus far, my non-payment rate has been closer to 25%. Most of these people are still late, and could in theory come current, but they also haven't made contact, so I highly doubt that will happen.
I don't think I have to tell you how quickly even a single loan, and especially 10-25% can wipe out your earned profits.
If everyone of these borrowers defaults and all of my other loans are repaid in full, I'll be at -5% for the year.