Also if you read more about the tool, you can see that it has native integration with others tools such as JIRA, so cool to be able to integrate a mockup easily when defining a new item...
Take a look to this screencast explaining the basic features and use of the product:
Another interesting tool is the online white board "DabbleBoard" that allows user to quickly create and share graphs, ideas, ... A very cool feature of this tool is the automatic recognition of shapes. The best way to understand, is simply to see it in action: