Just like banks and other players (ahem rating agencies) before the financial crisis assumed ABS were safe because they were diversified ("the housing market cannot collapse uniformly accros the country"), they can actually become very correlated in a very short amount of time.
So even though peer-to-peer lending is an interesting proposition, it is important to understand that it can potentially become ugly and sticky...
My #1 question, however, is what makes people turn to a source like this?
Peer to peer lending being somewhat new, I would say some of the borrowers think it's cool. Some of them might also be making a principled choice ("banks are evil"). But ultimately, if you have a reasonnably good credit profile it should be noticeably cheaper than the average traditional lender with its branch network etc.
The only bad thing about these sites is that your income counts as normal income and not capital gains.