We do. We are migrating our WPF, Flex, VB6 apps to our custom fw based on KotlinJS, ViewModels and bindable properties. There are some kotlin features whixh are not yet supported by JS compiler, ocassionaly something stops working with a new version of Kotlin plugin, but in general it is still the most productive frontend solution I experienced. We also generate a Kotlin domain classes from our Java backend domain objects to keep strongly typing when building UI.