A client asked me to quickly take a look at FlairBuilder. FlairBuilder is a cross-platform tool for rapid authoring of interactive wireframes and software prototypes. The 2¢ summary is I generally liked Flairbuilder but would not use it much.
I used it to create a simple two window interface where the second window pops up from the first. The interface creation tools were adequate to build the interface's look and interaction. Before building the interface I had a mental model of how the components were nested -- radio button groups within field sets within panels within split panes, etc -- but I could not directly apply this mental model to FlairBuilder's implementation model. While nesting is supported it is very rough in comparison to pure GUI builders like NetBeans' Matisse. There are no general nesting widgets and no split pane widget. Un-nesting is not possible as far as I can tell. (Perhaps it is just cut and paste.) All the widgets are fixed size and so my approach of laying out the widgets using their natural nesting and then tuning their positions and properties was not possible. Instead, you really need to have planned the interface before using the tool. Using the tool to explore is, I feel, not its strength.
You can create interactive models of the application. I find this less useful than showing specific scenarios in a storyboard presentation form. However, creation interaction within FlairBuilder is easy for a programmer to use. That is, the model of interaction as a series of "actions" responding to "events" has become natural to a programmer as this is how most GUI toolkits are implemented. However, this indirect model of interaction can be difficult for non-programmers.
The pixel perfect widgets -- radio buttons look like real radio buttons and data tables look like real data tables -- will inevitably lead me down the path of unnecessary precision in the prototype. This unnecessary precision would become a huge time-sink as I twiddle with widget properties, colors, and especially spacing. Given that there is no automatic widget sizing there would be lots of small changes necessary when re-aligning widgets.
I recommend considering Wireframe Sketcher and Balsamiq.
Update: See Cristian Pascu's response inRe: FlairBuilder review.