It's still early, but it makes Rx code much easier to read. So long as you're comfortable using Babel for transpilation, it doesn't really matter whether or not it's TC39-approved.
It'll matter in 3 years when you have an app that's full of rejected ideas.
That's the problem I have with the Babel-everything mentality of a lot of developers, they don't realize that they are essentially created their own quasi-language that they'll have to support on their own.
esproposal.class_instance_fields=enable
but it chokes on ones it doesn't understand. I tried using Flow and RxJS in the same codebase. RxJS makes heavy use of the pipeline operator:
stream::map(thing) vs map.call(stream, thing)
Flow will stop checking types as soon as it sees ::. Babel parses it, but Flow runs in parallel, so it needs to be able to walk the AST itself.