Create immutable state from a Class and get the best parts of an ORM for free. When I started with Redux I googled "Redux ORM" and immediately recoiled in horror. I just wanted to automate the translation between my state primitive and real classes. This gets it as a freaking byproduct.
Common primitives available on anything in microstate with no boilerplate. Uniform expression of these primitives makes translation to other forms easy.
Set nested properties in a consistent way without feeling like you're splitting the atom each time you need to do it. All (most of?) the operations just continue to work within Arrays.
Composable!!! Chainable!!! Constants!!!
Computed properties and transitions just fall out so naturally from using classes that it doesn't even seem like you did anything. Transitions take the place of many action creators in an obvious way.