XRP is 100% "pre-mined" and controlled by a couple of big businesses, so, anyone who invest there - are just participating in another yet Ponzi scheme.
Well it obviously makes little sense to compare XRP to a mineable alt-coin. It is more comparable to shares in a company stock. AAPL, EBAY (paypal), TWTR, etc.
IMO it is actually less ponzi-like than traditional company shares with IPO price-rigging, dilution, fraudulent accounting, and so on. And it is a lot less ponzi-like than some of the alt-coins which are raising funds on little more than overhyped proposals and lists of desired features - in one word: vaporware. While Ripple actually exists, has gateways, growing liquidity, and is already useful.
Hooray, let's replace the debt backed monetary system with.... an obligation backed monetary system! Wait.... that's suspiciously similar to debt based....
When you deposit funds to a bitcoin exchange, be it USD or BTC, the exchange is "obligated" to give it back when you want to withdraw (redeem). Its the exact same situation with ripple gateways (which is why some bitcoin exchanges are also already ripple gateways). Except on ripple you can trade and transfer deposits of any currency: USD, EUR, BTC, LTC... I love cold hard coin as much as anyone. But sometimes one wants to move USD around to different exchanges (e.g. to arbitrage price differences), and that's what Ripple does.