I've invested in P2P platforms in the UK for the last 4-5 years, and generally my experience has been positive. UK returns are much lower than what I see quoted on Harmoney, and perhaps this will moderate over time as more platforms and investors come into the sector - that is certainly what has happened in the UK to some extent. You can get perhaps 4-5% income from platforms lending to individuals, 6-7% on property, and up to 10-12% on SME platforms.
In terms of investment approach, most of my net worth is tied up in rental property all with (deliberately) high mortgages - and is therefore highly tax efficient but income can (will!) be volatile when rates rise. Therefore I use P2P (over various platforms) as like a big deposit account to provide fairly stable regular income, instead of dividend stocks etc. So far default rates have been quite low and the income rather good, and you can achieve very good diversification, so I would generally recommend it. The main problem with the sector is it is still very new and frankly has not been tested through a period of economic distress, and it is still rather unregulated (at least in the UK - but this will change soon in a big way as the government is now paying more attention to it).
Given most platforms are in the business of selling what are essentially very risky investments (e.g. personal loans, SME loans, etc.) to small investors - the sector will presumably will have a comeuppance at some point. But in the meantime hopefully we enjoy some good times. Diversify as much as possible!