"Mockups" isn't really the right term here as far as I'm concerned – it implies high fidelity, whereas this is a tool for rapid, low-fidelity idea sketching. It looks like an excellent wireframing tool, not a mockup tool.
It's just as fast to draw a browser window, button, or text box in Balsalmiq as it is to draw a rectangle in Precursor, but the former makes it unambiguous what I'm talking about.
The point of low-fidelity mockups is to make the conversation about the the overall design without focusing on detail. If "low-fidelity" means moving from basic web elements to basic geometry, it seems to me like that hinders the conversation more than it helps.
