I use Prosper. The single most important thing in P2P lending is diversification. The minimum amount you can loan is $25, and they highly recommend starting with $2500 - enough for 100 loans. That way you can lessen your risk of loss from defaults.
Prosper has a tool that lets you setup automated investing. You can define your criteria for loans and it will automatically invest your pool of cash into loans that meet your criteria. That's important because as you get paid back you need to reinvest that money.
I started out with $2500. Then I setup automated deposits of $200/month for 12 months. After a year of depositing money I turned that off and let my account simply grow from the interest I was making.
Currently I'm averaging a little over 8% yearly gains. In total I deposited $4900 and my current portfolio balance is $5704 after 2 years total. (+16.42% total). I pretty much don't even touch it now, my gains get automatically reinvested back into it and it continues to grow.
For me, it's just one more diversification. I have a maxed 401k and Roth IRA already, so this is just a way of investing money outside of stocks and bonds.
I used prosper from 2006-2008 just to play around a bit. This was before portfolio investing was rolled out and you bid on individual loans. Overall I put $358.63 in, and got $320.39 back, for a net loss of 8.39%.
I'm tempted to try lending club with some portfolio investments, but Prosper isn't doing business in Ohio yet.