We released Dart as a Technology Preview last year, which means we released very early in the project's life cycle. We've been collecting feedback and updating/refactoring the language, libraries, and tools. We've recently started releasing Milestone builds, which are essentially beta builds of the SDK and tools. The language does not anticipate backward breaking changes (but we still anticipate adding features like mixins). The libraries are currently undergoing changes, driven by feedback from the community and internal users.
The question if you should use Dart now depends on when you need to ship your projects. Too much will be changing to bet on it to ship your project in Q1 2013. However, my impression is that if you're shipping in Q2 or after, Dart should be a good bet.
The Dart community and team is standing by to help if you have any questions or concerns. You can find us at firstname.lastname@example.org, dartbug.com, and Stack Overflow.
Dart is targeting HTML 5 capable browsers and ECMAScript 5 capable browsers... which tends to means stuff that is current as of today. As google tries to run on some older browsers as well, dart is not a good solution.... yet.
There are teams within google that are starting to build sites with it, but nothing is public yet.
The time is good to check out Dart and let us know what features you need for your next commercial project. However, with the libraries evolving right now, I probably wouldn't use Dart for something I need to ship in Q1 of 2013.