Immer is excellent. It makes deeply nested immutable updates a breeze. The API almost makes it feel like you're programming in a fantasy-land future where JavaScript's default data structures were persistent data structures like a proper functional language.

And it accomplishes all that without sacrificing type safety like most string path based approaches to deep updates often tend to do (think lodash.set or ramda.assocPath).

The only reason we haven't gone all in on it yet is the existence of browsers that don't support Proxy (like IE11, which we now unfortunately have to support), where performance takes a huge hit, and those tend to be the browsers that can least afford to take that performance hit to begin with.