Like a ton of other people here, we've been using ReasonML for about half a year now at Rolltrax [1] and so far I can say I'm ~95% happy with it. I'm a long-time OCaml user but my co-founder had 0 experience in OCaml when we decided to make the switch to Reason. I'll just list off some things we've found using Reason so far:

Positives:

* Really quick to get productive -- the syntax really does do a great job getting new users familiar with the language constructs (I was very skeptical about this before)

* ReasonReact is very nice and works great out of the box

* Writing bindings is generally pretty easy to do, and we're currently maintaining 4 of these (+ 2 existing binding we've contributed to)

* The language and compiler do a great job at working with you to figure out issues

* Everything feels a million times more structured than normal JS code

* Not sure if this is an up-side, but writing code in Reason is so much smoother than other languages that I feel like it's pushed us to implement more client-side features than server-side ones (unfortunately, this means everything depends on a ton of JS)

* Super readable, I feel like I can skim the code and know exactly what it does instantly

Negatives:

* It's still JS, which means you have to deal with a ton of crap from NPM, WebPack, JS's module system, weirdly written libraries that don't do well with bindings, and things breaking in weird ass ways that you can't easily debug... I think this has been the source of 90% of our bugs so far

* Still no way of getting higher-order components working well, specifically with bindings, which makes it really hard to work with libraries like react-grid-layout

* Polymorphism could still be improved. We occasionally get type signatures that should work in theory but can't because of the object system's limitations, which means we have to give up on the type safety and fiddle with Obj.magic or external JS code

* The JSX syntax is still annoying because you can't use spread syntax with a native element (e.g. div), although eventually I'm gonna get tired enough of doing that that I'll open a PR to fix it

Overall, Reason is by far my favorite way of writing web apps. If that's something you've got to do, give Reason a try. There's still a few pain points but interop with JS is so simple that it hasn't really slowed us down (except when webpack breaks lol)