At the beginning of a video by Vitalik Buterin on scaling he discussed the speed of the Ethereum network and its important characteristics. I highly recommend watching this video (at least the start). In it he describes how Ethereum is presently 1,000,000 times slower and more expensive than regular computers available today. Even once scaling works Ethereum may still be 1,000 times slower than a normal computer. So why use it at all?
Next he describes 5 features of a blockchain that justify its use. If your use case does not lean on at least some of these features then you have to consider it is a useful blockchain implementation. I will score all Use Cases on this website based on these features.
Censorship Resistant – The contents of the blockchain cannot be changed or blocked, not by a government, a corporation an individual (or even the author). What is written on the blockchain, stays on the blockchain. The contents are \immutable\.
Fraud Resistant – Removes the requirement today to deal with a trusted intermediary (such as Mt. Gox, another financial institution or Facebook) which is capable of acting fraudulently
Transparency – Any member of the public can see the transactions taking place on the blockchain and know that the transaction they see are the complete transactions
Robustness – as the Ethereum Network is composed of 16,000+ nodes (as of July 2018) which are independent and spread out geographically (in other word decentralized) and almost all of these node must fail for the network to cease functioning, the Ethereum Network can be considered robust
Interoperability – Interoperability is the least obvious of Ethereum’s key features but it has specific advantages. There is a public way to any computer program to communicate with the Ethereum Blockchain and its DApps. This interoperability is also immutable. It can be improved, but never removed. This makes the Ethereum blockchain immutable in its interoperability. This is unique and useful. It you build software that works with Ethereum, no one can arbitrarily take away.