I have made some really good returns on BTCJam, but you have to really research and study your loans.  I did fairly well on Prosper and a few other USD P2P lending sites, so I applied some of the same principles to these.  It was a lot risky then those sites, but I made a better return. 

I think there are too many people who see a 200% return on their investment and think this who I need to invest with only to lose their money.  If you follow a set of guidelines and not chase the biggest riskiest investment, you can make some serious BTC.  I think a lot of the complaints come from a lack of research and analysis in P2P in general and with borrowers.