I've been holding out spending any extended amount of time on Dart until 1.0 as the API for it kept changing fairly dramatically since it was first announced (it was first announced on October 10, 2011 at the GOTO conference).

There isn't anything super amazing about this language from a language perspective, but I think you need to look at what Dart really brings to understand why it may be a nice alternative to JS.

* Optional typing. While typing data is completely ignored at runtime, even in the DartVM, it can help a lot for tooling if you are working on a large project. If you don't care about typing, you can just use "var" for everything.

* Batteries Included: IDE, package manager, core libraries. This means you get decent IDE to start coding in. If you don't like Dart's eclipse-based IDE, JetBrains has been maintaining a Dart plugin for their IDE (IntelliJ or Webstorm). The core library includes a lot of useful classes (not to the level of Java, but definitely has a lot of useful default packages). The package manager is nice to ship with the product as well to maintain use of 3rd party libraries.

* Nice development story. Sure, no shipping browser has the DartVM in it, but when you are doing initial development, you can use Dartium (chormium build with the DartVM in it) to do fast, iterative development. When you need to cross-browser test, you deploy to JS and see how things go.

* Speed. If/when the DartVM starts to ship in Chrome, it should help performance of apps that ship as Dart code. Specifically the runtime performance can be better as their DartVM doesn't have to support a lot of legacy syntax/behavior that JS VMs have to worry about. Along with that, the DartVM supports Snapshots. These are basically a heap dump of an application right before main() is run. This allows Dart apps to start up a lot faster on subsequent reloads.