This seems like a great tool specifically for prototyping as opposed to the tools that try and also create mobile web apps or functional prototypes. I have tried http://tiggzi.com/ http://codiqa.com/ and a few others, but have found none of them good for practical use. Most people I know use Balsamic or just Photoshop, but this seems good for rapid prototyping.

Suggestion: In preview mode, is there an easy way to prevent users from dragging the images on the buttons or prevent the preview from being drag selected and looking like selected text?