Typescript's single biggest win for me is the ability to provide type definitions for my models. This is where the vast majority of my JS bugs come from -- somebody trying to access a nonexistent property of some object (or, if it does exist, incorrectly assuming the type) which was serialized from JSON. Better yet, if you write your back-end in Node you can use the same models in your rest endpoint. It's a huge boost to both maintainability and developer productivity.

However, my biggest frustration with Typescript is the general unreliability of @types. The odds that a given set of typings are complete, correct, and up-to-date are basically nil -- even for some relatively popular projects. Sad to say I've become an expert in Typescript's crazy module augmentation nonsense. Hopefully more big players like Slack contributing to DT will help.