Another problem is that it needs gateways like exchanges by bitcoin and actually there only one gateway.
Without gateways to fiat you cannot transfer fiat money just Ripple.
You can also "ripple" IOUs in any currency through your private trust network. Eventually gateways will become hubs that will connect everybody easily, but you can get started without them. It's a pity the existing gateways only accept IOUs for the USD and not for other real world currencies.
A virtual currency based on USD or EUR already exists. Liberty Reserve (USD and EUR) is easier than Ripple and it is also centralized but at least you don't need to install a closed source software where could be anything.
Ripple is an overkill for this purpose.
A total decentralized virtual USD could be constructed with the bitcoin technology with colored coins on the top of bitcoin or namecoin.
You do not need to install closed source software for ripple, the client is open source. The server is currently closed source but they intend it to be open source. They also intend the ledger to be decentralized. That's their stated intention and we have no real reason to doubt them at the moment. Liberty Reserve do not have this intention.
I don't see how your decentralized vitual USD with bitcoin would be better than ripple rather than worse. Each Gateway/Exchange/Whatever would have to issue their own coloured coins, and there would be no decentralised mechanism for exchanging one Gateway's coloured coin for another at parity or otherwise unless one is built separate to the bitcoin network. I haven't seen a solution for this in the bitcoinx documentation, can you describe it?
Thrust network:
If you thrust in somebody then why do you need Ripple to intermediate ? If you friend or your neighbor want to borrow 1.000 $ from you why would you need Ripple for it ?
A more complex thrust network for an organization or a religious group can be built more reliable with colored coins on the bitcoin or namecoin network as stated above.
I agree, and the free transfer of debt that they allude to on a social scale is difficult to see ( Trust is A -> B -> C, so when C decides to send A and IOU it ends up with C owing B and B owing A, a frankly ridiculous position for B to be in. Unless, of course, B was being paid for this service, but then we're talking about business not friends). Some of the examples they give on their website, such as the restaurant one, while intending to be helpful are actually just confusing and give the wrong idea.
So the problem which exists by bitcoin with exchanges by Ripple is even worse at the moment it is just hidden by the complexity of the system which is difficult to understand.
As far as the mechanics of the payment system go it is better than Bitcoin, but it suffers from the same lack of adoption by real world merchant. However, I expect it to grow much more quickly since it can provide cheaper and more convenient payment services than traditional systems in any currency. The extreme volatility of BTC is a major impediment to its adoption as a payment system. Ripple allows you to avoid that risk by using fiat currencies. Adoption of XRP might also be easier because I've heard they are trying to stabilise the exchange rate, which they can do to a degree because they hold the bulk of all XRP that exist and because XRP reserves are needed to activate accounts and use their functionality.
But the killer app for Bitcoin users is as a decentralised exchange for BTC.
I am not sure if it would be more decentralized than a Liberty Reserve - Bitcoin exchange. As long as Ripple is controlled by somebody he can be forced to cooperate with the authorities.
The remaining XRP that haven't been distributed would be controlled by Opencoin but everything else would be distributed. Lots of trusted people would have to be forced to cooperate and even then there's trust agility so trust could be assigned elsewhere.