>Are Bitcoin or Ethereum really good as a currency, though? They're quite volatile.

They're mostly used as speculative capital trading right now, and can't be compared to any traditional currency in terms of volatility. If the dollar was magically swapped for one of them in an instant, and we all carried on transacting with it like we did with the dollar it would be a different story.

Volatility isn't an inherent property of a currency. It's a function of market dynamics.

(I'm not an economist so excuse potentially poor terminology and I hope I'm conveying this well)