I really like Elm and have become an intermediate user of it (as well as Haskell). I felt like I really understood what Elm signals are about and made some optimisations to Helm [0] based on that. I pushed to use it for a contract for an open source application [1]. Ultimately that wasn't a success due to various factors but it's interesting to note that the client dropped Elm after I left and hired people to re-writ…
The JavaScript compatibility problems across browsers are already enough, for me to sell to customers yet another layer to debug that might not even be properly supported by the respective browser debugging tools.