We use CoreOS rkt in as developers and as an organization. My department uses CoreOS rkt to compartmentalize all of our developer operation tools and to keep systems homogeneous across hardware. My organization as a whole use CoreOS rkt to containerize and scale customer instances.
As a developer CoreOS rkt is a lifesaver, it has allowed us to write tools focused on developer productivity and use them across the department regardless of underlying operating system. We have hundreds of compilers, linters, and unit tests across dozens of languages stored in CoreOS rkt.
As an organization we use CoreOS rkt when spinning up new customers. This has been advantageous because we can run multiple customers from one VPC. It also means we can scale up a customers environment almost instantaneously. We often have customers who's resource needs change on a daily basis, CoreOS rkt has allowed us to scale with our customers.