You wouldn't choose it over haskell every time, but for the limited subset of times where you need to write frontend code ReasonML has distinct advantages.

ReasonML is basically OCaml that compiles to JS, designed to make interoperation with Javasascript easy. There are compile to JS projects for Haskell too, but the ease of interoperability with popular JS libraries like react and support from facebook's open source tooling chain make it a much better choice for most frontend projects IMHO -- unless you're already a haskell expert in which case go with what you already know!