I'm about 95% sure Bitfinex is not a ponzi. I am lending a considerable amount of money there. Here's my reasoning:

1. Interest rates move up and down along with lending volume. When no one wants loans, the interset rates drop. When BTC rallies, loan volume shoots up, all available lending offers are used up by BTC traders and interest rates spike.

2. There are other humans competing against me to lend. If I set a rate at 55%, in 5 or 6 minutes some bugger will come along and ask 54.9%. Either they have an AI-type software that is programmed to compete against lenders or there is a real human on the other end trying to undercut my lending offers.

3. The site operator accepts deposits by wire transfer. If this was a ponzi and he disappeared, I could take legal action against him and I would have proof of sending him real money (not just play money, i.e. BTC).

4. It makes sense to pay 100%+ per year to buy bitcoin if the price is rallying. It makes sense to pay 0.3% per day when bitcoin is going up 10-20% daily. Bitfinex lets you borrow at 5:1, so even a 1% upward price move gains you 5%. With a 0.3% daily cost, you can still make money. So, I don't think traders are acting irrationally by paying these margin rates (as long as the bull market continues).

So, all these factors have convinced me the site is real. For me the risks for bitfinex are the following:

1. The site grows so big with so many deposits it gets tempting for Raphael to run off with all the money.
2. Theft and/or hacking either internally or externally.
3. Raphael disappears and no one can process withdrawals.

Bitfinex is still in beta and so far the people running the service seem interested in running an honest and legitimate service. Let's hope that continues for the long-term.