I have a little BTC and ETH just for fun.
It 'tumbles' and then 'skyrockets' quite frequently. Incredibly volatile.
I bought in at $16 for ETH right after the EEA announcement, pulled out my principle at $54, and now I'm just watching this crazy roller coaster ride. It's fun.
But I sure as hell am keeping my retirement in my 401k. I cannot imagine the stress on people who have a substantial investment in BTC or ETH, but they probably have a totally different mindset than I do when it comes to cryptocurrency.