Domestic P2P lenders like Lending Club and Prosper may be able to shave a fraction of the cost of borrowing within the US. But I think the real windfall to be gained from P2P lending is where the local alternatives are most expensive and inefficient.
For small loans to low-income people in developing countries ("microfinance"), the global average interest rate is about 40%. It is not necessarily profiteering; their model is just incredibly inefficient.
Even zero-interest Kiva loans cost more than 30% on average to the borrower, simply because the local banks that manage the loans need to charge that much to cover their operating costs.
Zidisha bypasses all that by offering a do-it-yourself microfinance marketplace. Our operating costs are roughly 5% of the value of the loans, versus the 40% or so that is traditional in this sector. The savings get passed on to borrowers and lenders, who are free to negotiate the interest rates they find appropriate for each loan.