Works very well with a Magic Trackpad. Only two gripes- its annoying how when you choose to save your signature to the desktop it erases any previously saved autographs (any PDFs that are called "autograph"), and its sometimes hard to sign because its hard to estimate where your finger is. Maybe itd be better if for the Magic Trackpad the window changed to a square size to match the size of the trackpad? Otherwise its a great app, well worth $3!
gizmosachin about Autograph