The all time fully repayed loans for the site is increasing gradually, last stat was about 76.94% of all loans that has been taken out have been fully repayed and then there is some loans that is partially repayed and some of them is still active but not overdue so the total repayed on individual payment level is probably in the 85-90% level.
Its always recommended to diversify the investments to some degree, calculations from the fiat P2P lending world showed that one needed to make 146+ investments(but for example 500 was better) if investing equal amount in each investment to be garanteed to have +ROI on lending club, as the default rates is slightly higher and the interest rates is higher in the BTC P2P world its likely that the diversification needs to be larger so in the region of 500-1000-2000 investments will probably be needed to give a equally stable result. If one dont invest blindly in just any loans but choose wisely, then ones investments should be able to outperform the sites avg result a lot.