XRP is a very controversial token. In these questions, I am answering about this token and I am trying to be impartial.

... centralised?

Yes. It is not, cause most of the tokens own Ripple Labs (XRP are mostly in escrow and every month they get 1 billion XRP). It is the cause of nodes. People can make as many nodes as they want, but even Ripple Labs. says, that people should trust some 3rd party node, which is not safe, cause that node can scam you. Better is to make your own. It is almost free, but it can have a few tens of GB.

... sustainable?

No way. This is one of the biggest problems of XRP network. All coins are pre-mined, so any others cant be created. But when you make some wallet and send some XRPs to it, you have to let 20 XRP in your wallet and also when you are paying for the transaction you have to burn some fee. There are no miners and that means, that tokens which were used for fees are just burned so there is no ecosystem - reusing that coin. That means, that it can't be forever, cause it will gradually lose XRP tokens and at the end, there will no one left by which we can move - but it will take few hundreds of years. 

When I was reading Wikipedia it looked there as Ripple is proud, that XRP id deflation token. But in the next 20 years, it will have extremely huge inflation until all tokens will be out of escrow.

... good investment?

I don't think so. Ripple Labs. are getting every month from escrow 1 bln. XRP so if they will sell it at the time of writing this post price will go down by about 3%, which will not make you happy. Also now is inflation so high, that people should wait with investment in that. Also, I read a few predictions which were saying, that XRP is overpriced even it was at 0.30 USD.

Also, I don't like some small coins/tokens, cause I don't like to look at so many decimal numbers and easier is for me to look at Bitcoins graph.

... fast?

Extremely. Once I was trying to send money from 1 exchange to another, with a "stop-over" at my wallet it took only a few seconds. The problem for me was, that it took me a few minutes until I found block explorer, cause I still write "XRP blockchain", but easier it is to look at "XRP block explorer".

Also, this blockchain can have much more than 65k transactions per second which will be at the same amounts as Visa. But I think that at the shops people will pay also with another crypto, which have extremely fast blocks, cause it will be stupid to wait a few minutes until the transaction will be included in the block as at BTC or LTC.

... good for banks and their transfers?

XRP has good technology and they are using so much of their money from escrow to be popular at the banks. I think, that they must be first, cause I am sure, that it is created much better blockchain, but Ripple Labs. will at that time have huge banks at their side. But I think, that banks should use more blockchains, cause XRP has the same features as another crypto for regulators - they can't do anything to stop the transaction. And when banks will use also ETH or XLM it will be the same. But now are only a few banks are using this technology and they surely have fees same as it will be an old transfer of money.

But in Europe, we have Bank secret laws and when banker showed a statement of account of Minster of the interior he went to prison. XRP is a public technology and that means, that XRP will have to become privacy token or memo will have to be changed whenever a person gets the money.

... making airdrops so the whole money from escrow are not going to Ripple Labs.?

OK. Ripple Labs. have to make some airdrop, cause ledger will not do it and someone will have to give that money to escrow and send it to approved ones. I got this question, cause XLM is doing it. They have SDF (Stellar Development Foundation) and they are few % of the whole amount are sending to some good and mostly decentralised apps which are using that blockchain in some way. You could earn a few tens of USD in XLM and they have good marketing. 

I think, that XRP should do some of these airdrops and give away a few USD in XRP - but with some verification, so people will not to do that twice (but not KYC). People will use Ripple ledger and even some people who hated it will use it, cause they will find out, that it is OK.

... used as stablecoin?

Technically yes. Ripple has 3 technologies which are running on one ledger. One of them is xRapid. 

xRapid is like some middle-man at interstate money transfers. The person is sending money from the UK to his brother in France. The person will give GBP to the bank and the bank will change it to XRP. XRP is sent to French bank and there is XRP changed to EUR. And this money will be credited to the brother's account. 

At this way, it is used as stablecoin which is not good, cause there can be a drop by 0.5% at that 4 seconds, that will mean, that for every 1 000 USD he will loose 5 USD. 

So why is XRP not used as stablecoin?

I think, that if it will not be stablecoin Ripple will have to find from somewhere every month 1 bln. of something to release it. But when it is this way Ripple Labs. can just sell it and take how much they want.

... managed mostly by XRP?

At XRP we know the creator and that is the problem. If he will make-up some stupid thing I am sure, that most of the network will stay by him. At Bitcoin, there is no one who we know, that created it so when there are some change miners and nodes will think about it and if it will be a good change network will support it. I think, that Ripple Labs. should make some voting at some changes, cause I don't think, that everyone will not agree with everything.