Is it really a cryptocurrency though? It seems more like a paypal alternative (i.e. the ripple foundation can roll back any transaction at will and has done so in the past)

In all fairness, so has the Ethereum Foundation with the DAO and the latest Parity hack is not unlikely to be next.

The difference is that the DAO hardfork was not done by the Ethereum Foundation. It was recommended by the Ethereum Foundation and the actually done by the vast majority of miners, in consensus.

No, seriously; which transactions were reversed? I'm unaware of any such thing occurring on the network.

The construction of the network/consensus protocol means that Ripple would have to get a majority of all validators on the majority of everyone's UNLs to lie to reverse a transaction. Ripple likely does not have such influence /or/ if they do, then you are free to select a larger UNL set, along with the rest of the community, such that they /do not/.

> has done so in the past

Which?

Ripple is a permissioned blockchain.

I don't think it is a blockchain. Google ripple consensus protocol.

It is a Merkle DAG of transactions secured by cryptographic protocols. It very much is a blockchain -- it just doesn't use proof of work as a mechanism to exclude bad actors. The fact that it uses UNLs and consensus to achieve a consistent view of valid transactions does not make Ripple "not a blockchain" and, as XRP relies on cryptographic mechanisms to secure rights of ownership on that blockchain, XRP, the native Ripple asset, is by definition a cryptocurrency in the colloquial sense.