I have used Prosper for several years, and it has its ups and downs.
I lent out about $1,000 in $50 increments to AA/A/B credit borrowers, with good fundamentals, but even some of them have defaulted.
The loans that did not default have produced strong gains (avg. around 12%), but the defaults kill all profits.
As a borrower, I think its great, but as a lender, I would not recommend it.