While this may be a great tool for the scanner deprived, I have used a scanner for this for doing this for many years. Just sign a blank piece of paper; scan it; then save it as JPEG or PDF. Then just copy and paste the "document" into your message or document. I keep mine in iPhoto for faster access from Apple applications as almost all of them can load up an iPhoto item directly. One a signature is in my document I am usually able to resize it to fit the need.
While this product is not expensive or necessarily out of line cost wise, my feeling is, having a scanner, why spend $7 on this when you might be able to spend it on something else more useful?
Not with any intention of casting aspersions on this product as I did not even download it, but why spend money on a another tool when I have one that already does the job effectively for me, unless it can do significantly better or easier to make its additional cost and maintenance worth while. This is a factor I always consider when considering the purchase of any product.