Using it on some new functionality at
http://rplan.co.uk, pretty pleased so far. The language is great, with some really neat features such as using interfaces for structural typing (
http://en.wikipedia.org/wiki/Structural_typing) and declaring properties in the constructor definition. The ability to progressively type code is obviously quite nice too. Coding speed is typically quicker than JS because the compiler catches bugs that might take longer to surface (especially when integrating components.) Refactoring is also easier thanks to both the type system and tool support.
The compiler speed is OK (well, 0.9.0 wasn't, but 0.9.1 should be back to where it was before.) The main pain point is that we usually want to type any 3rd party JS we interface with (e.g. Angular, Backbone, Highcharts etc.) though https://github.com/borisyankov/DefinitelyTyped is helpful with this. No major complaints as of yet.
It's still perhaps too early to tell how long MS will be supporting it. Windows 8 has a JS interface which is comforting, but they could turn around and decide to stop development on it. That would be a great shame, but the code is open source, the compiler works well as of today, and the generated JS is readable enough that it can be edited.
We're slowly migrating over from Coffeescript.