I've only played around with this tool for a short bit, but it immediately became apparent to me that it kicks ass. I've always been a believer in paper prototyping, but, I have to say, this tool might actually be better than paper.
The interface feels very smooth and intuitive and responsive. The expansive widget library actually seems to provide a mechanism whereby you can visualize a page layout faster than with pencil and paper. There's no erasing mistakes - you just move or delete the offending element/widget. That, combined with complete undo support, really let's you get at it quickly and fearlessly.
Having the sketch available in digital (rather than paper) format is very compelling. Paper is fragile and easily lost. Digital allows for backups, versioning, sharing, emailing - you know the deal.
At $79, the product seems "well-priced", which you have to love.
Bottom line - wow.
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