That blog looks very interesting.
I ran some numbers on interest rates and average default rates for the different credit grades of borrowers on Propser, and came to the conclusion that you'd only have to have slightly worse default rates than the posted average, to do worse than traditional savings, or actually lose money.
In other words, if 20 out of 20 "C" borrowers pay in full on time, you've made a great return. But the odds are at least 1 will default, so you really can't count on that full amount. If you're unlucky and 2 default, you haven't done any better than, say a CD getting 5%. If 3 default, you're losing money. I don't think those are very good odds, really.