<p>Of course the best metric would be a happiness of your users.<br>
But what metrics do you know for GUI usability measurements?<br>
For example, one of the common metrics is a average click count to perform action. 
What other metrics do you know?  </p>

Jakob Nielsen has several articles regarding usability metrics, including one that is entitled, well,
Usability Metrics
:
The most basic measures are based on the definition of usability as a quality metric:
success rate (whether users can perform the task at all),
the time a task requires,
the error rate, and
users' subjective satisfaction.