Ripple is very difficult to understand and therefore is difficult to argument against it.
Unfortunately, that much is true, for Ripple as well as for Bitcoin.
But what I understood it is centralized and not open source.
The architecture is decentralised, but there are only a few servers right now if I understand it correctly. There are instructions for the Ripple server (rippled) on the Ripple wiki, but I haven't been able to download any binaries. I'm not sure if they have been released yet, and I know the source code hasn't been. I imagine the binaries haven't been released yet because the system is still in flux.
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.
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.