How is this different from using Facebook Flow? I have been using Flow on a JavaScript project, along with Babel for ES6, and enjoying the experience quite a lot.
Flow is also based on a daemon which continuously monitors your code. As such, it incrementally checks your modules allowing it to be extremely fast once started.
I ended up using flow since it fit with the language feature set I was using with Babel (JSX in react code being one example). Since then, I've grown to enjoy the flow based typing and the rather good support for optional and non-optional types.
Looking at the code lately seems to show some interesting stuff in the pipeline too. Taint tracking seems like a very useful one that I'd love to implement and seems like a natural extension of the code-flow based type system.
Flow could definitely use more documentation and more example code but I found the community on IRC extremely helpful and polite so perhaps it's not a bad time to start if you're willing to do a little reading on the site first: http://flowtype.org/docs/getting-started.html
TypeScript also has a language services server which runs as a daemon, lets you submit incremental edits to it and supports services such as autocomplete, rename refactoring and pretty much anything you see in visual studio or visual studio code.
But yes, the type system has a couple of unfortunate flaws that are relics from its beginnings (cant express non-nullable types).
TL;DR: Flow inference is super nice but community and tooling is lacking compared to TypeScript imo so we ended up using TS