These tools take too much time to use and force designers to specify more of the design details than they wish at this stage.
Most designers prefer to sketch early interface ideas on paper.
I have developed an interactive tool called SILK that allows designers to quickly sketch an interface using an electronic pad and stylus. However, unlike a paper sketch, this electronic sketch is interactive. The designer can illustrate behaviors by sketching storyboards, which specify how the screen should change in response to end-user actions.