I use TypeScript on a medium-sized (>50K LOC) single-page web application, as well as on some personal projects. The large project was ported over from JavaScript, which went quite smoothly. So far TypeScript has been great. Doing front-end web development while writing type-safe quite is a nice luxury I have not had before.

The only thing I would complain about is some of the tooling is still a bit unstable, which is understandable. I'm using Visual Studio 2012.

I've been looking forward to 0.9 for some time now, as the generic support allows me to fully express all of the types in my system.

To me the most beautiful thing about TypeScript is it's compatibility with JavaScript and how definition files work. Definition files essentially allow you to write code contracts against existing pieces of JavaScript code that may or may not be outside of your control.

https://github.com/borisyankov/DefinitelyTyped is a good resource for definition files.