Has anyone used a 'social lending' site?

I never knew of this type of facility until someone on the Advice forum recommended them to an OP who may have some spare cash. Basically you lend your money to others and I guess you receive a rate of interest through the repayments.

On a quick search I found this site as an example. To anyone who have used these types of schemes to either lend or borrow from, what was your experience? This site's reviews seem to be a bit hit and miss.