The tech is interesting and the ideals of a currency not aligned with a government or big bank are nice but cryptocurrencies have no real value.
Essentially, they are a verifiable ledger of anonymous people exchanging bytes. They may have paid for those bytes with a real currency, much like someone paid for tulips some time ago, or they may have won them for verifying something on the ledger.
Some people in the tulip craze actually had tulips though, right? If a cryptocurrency falls to $0 USD exchange then all that is left is a digital ledger.
"At the peak of tulip mania, in February 1637, some single tulip bulbs sold for more than 10 times the annual income of a skilled craftsworker" [1]
I wonder if time of year plays into the psychology of this.