I have it set as my default shell on my main machine, but it's not going anywhere near my servers! It's definitely the most fun shell I've used, and hopefully some more of the warts can be shaved off as it matures.
It's not stable and therefore introduces breaking changes. They clearly mention this on their README.
This project has reached a minimum-viable-product level of quality. Many people use it as their daily driver, but it may be unstable for some commands. Nu's design is subject to change as it matures.
Had a discussion about this with one of the Nushell contributors, I am conflicted about it. I think the idea of it is somewhat amazing but also somewhat horrendous. I think the concept of this is a step in the right direction in terms of evolving the shell into something better than the traditional shell with concepts from more modern shells, but still has a way to go before adoption is more widespread. My 2c.