I've dabbled with Ratesetter since they launched in Australia.
I think they're a pretty good company. The returns you will get on there are far higher than any savings account - especially if you can lock some in there for the longer term options (3-5 years).
I am waiting for the volume to pick up a bit though, before I become more heavily involved. I understand that if you're lending in the 1 month or 1 year markets, your money can actually be locked in there for up to 1 year, or 2 years, respectively. This will only happen if they're unable to replace your lend amount with a new lender (unlikely but who knows right...).