There is a simple example that shows how to get started.
The js_bridge package includes a performance benchmark. It measures the time required to complete a single function call with a single integer argument and a single integer return value using both bare dart:js and js_bridge.
Below are the benchmark test results running on a 2.8 GHz i5 with OSX 10.9.3, Dart 1.4.3, and Chrome 35.0.1916.114.
Dartium (Dart VM):
Does it really work? Is it actually useful?
Yes. It is used in a real life project to put an ExtJS interface on top of a layer of Dart code that is responsible for all server communication, resource caching, etc. It works fine in this application.