For all the cases I have checked the type checker is extremely impressive. I have even checked whether types flow through promises! and with underscore.js and it has not failed me yet, it is extremely robust.

But aside from that, the most useful feature of typescript to me has been the productivity gains, just checkout: http://www.typescriptlang.org/Playground/ You can now have an IDE that provides useful information about clases/objects/etc. (imagine if javascript actually had proper codecompletion and intellisense)