From the borrower's perspective, if you can't qualify for a traditional bank loan, they were amazing. (I fit that description at the time: there was a dearth of traditional banks which were well set-up to service a young white professional whose salary was paid in yen.)
My loan was one of the success stories: I couldn't have gotten a student loan through a traditional lender ("Your brother is going to cosign this? From Japan? Via fax? Pull the other one, kid, it's got bells on"). My peers, other lenders from the official forums (which were later closed), trusted me after several months of online interaction. I asked them for $5,000, they crowdsourced it in $50 and $80 and $230 increments. They got all their money back and 13% interest besides. Prosper got $50 and 1% APR for providing the platform. My little brother got to register for classes at Notre Dame on time. Everybody won.
And for a while, I thought loans like that were going to be common enough to paper over the (numerous) faults. I was catastrophically wrong.