In reply to Removed UserBoingBoing:
I keep a fair chunk of my excess cash in P2P; probably a higher percentage than would be advised by a financial advisor, but it's money I can afford to lose if it all goes horribly wrong and the returns can be very good.
I don't use Zopa - they had a period a while ago of not taking new deposits because they were awash with investment, so I just ignored them.
Ratesetter and Assetz (via their instant access and one month access) are comparatively pretty safe, but consequently low earning. Growth Street is a little riskier but a little better paying.
I'm not at all a fan of Lendy; perhaps I've been unlucky, but the fraction of my loans that have been indefinitely held up through in defaults or extensions is enormous. As they finally come back (usually because the asset has been repossessed and auctioned) I am taking my money out of the site.
MoneyThing has been brilliant - higher risk, but higher rates of return, and I've only ever experienced one default. As my money comes out of Lendy this is generally where it's going.
Overall, I've had a return of about 9%, which will go up a bit if/when the remaining defaults in Lendy ever get recovered. If you're thinking of this, spread your investment over several platforms because they can fail (I was lucky and lost almost nothing when Collateral collapsed earlier this year). I found financialthing.com a really helpful comparison site.
Oh - if your potential investment is more than £1000 and can last for a full year, Ratesetter offer a £100 sign-up bonus at the moment. This actually makes them one of the best paying as well as safest platforms there is - you'll probably make about 13-14% overall on that £1000 in the first year.