I've been on it for a few months now. You can get a decent welcome bonus if you sign up with someone else's referral code, so my SO and I both set up accounts :P
Our investments are split 80/20, so we aim to have half of the 20% in VAF and half in Ratesetter. Right now we only have about $1.5k in RS. It's a risky investment, but we are early in accumulation phase...TBH, I see it as similar risk to other investments - I think the big real risk to your capital is if there is mass and sudden unemployment and loads of people default at once.
I was annoyed at xmas time, as it took a couple of weeks to reinvest some money. Nothing was being lent on the platform over public holidays, and the 5-year rate dropped to ~9%.
I have it set up to auto-reinvest, but it can just sit there. Sometimes, it goes on the market at the current market rate (e.g. 10%), but then immediately, the rate will drop lower, say 9.8%. Even though my reinvestment settings are fine with 9.8%, the money just sits there until the rate goes back up!