I would add caution to some of the returns being discussed. In all "investments", your final return is directly related to the amount of actual risk you assume. High returns, though there are exceptions, are generally the result of exposing yourself to higher risk.
http://www.investopedia.com/terms/r/riskreturntradeoff.asp
Ironically, Prosper.com debunks this "economic law" in that their riskier loans actually have terrible returns on investment. The high interest rate loans, if their data is correct, are a gamble. Actually, blackjack probably has a better rate of return if you factor in the free drinks.
For reference, see Prosper.com's own figures of return on investment (ROI) after factoring in their (short term) historical default rates.
http://www.prosper.com/lend/performance.aspx
If you follow the link, and click on the link under the "Estimated ROI" section, Prosper.com says that the historical return has been somewhere around 6-7% annually for the better credit grade loans (AA, A). That is before taxes. The loans for borrowers with poor credit have had disasterous returns, due to the sky-high default rates.
Another caution unique to this type of business is that during tough economic times, higher risk borrowers have a much higher rate of default. This is evident in today's housing credit market. If the economy heads for a recession, these people will be the first to default.
My suggestion is to stick to what you know, invest for the long term, and expose yourself to risk when your expected return (over time) justifies it.
If this was such a good deal investment, the large professional money managers would be jumping all over it. They aren't, because better returns can be had elsewhere for less risk.
YMMV.
