The current volatility in the market makes it very dangerous for unassuming investors.
"Ethereum... which had seen its value rise fiftyfold since the start of the year to $300 a coin"
ETH (Ethereum) dropped to nearly 200 dollars per coin from a high of nearly 400 dollars, went up again to 250 and is now around 270 on my exchange. This is in a space of a week.
The speculation is based upon future projections of use but the block chain technology of four years ago, or more, such as BTC or ETH has become dated. Updating it is cumbersome. So, it would be easier to start new currencies for new projects, especially if there are easy to use exchanges to exchange the currencies. The market share of BTC has already been dropping considerably.
I would recommend that no one makes any medium to long term investments in the classic cryptocurrencies, perhaps small amounts on new start ups if one researches the technology and foresee it being scalable.