I don't get the magical power of a signature at all. Everybody can write my name under a document. This project basically allows you to forge your own signature. Is it still legally binding? Do these rules even remember the original intent?
The question is, can you forge your own signature? If both parties agree that the document is legally binding it seems a bit unlikely that the document would fall under the forged label. Though I am not a lawyer. It's also worth noting that digital signatures throughout the european union have legal status.
Until society catches up and uses cryptographic primitives provided by a national ID smart card (such that Estonia has) for authorizing intents, this is a satisfactory method to make document execution less painful.
This project is already doing the easy part (“place pretty signature picture here”). Depending on your jurisdiction and their tolerance, you could also render a true crypto signature in ascii-armored format to assist in proving legitimacy (perhaps generated as a small print signature line under the signature).
Sidenote: Some transactions require a "wet" signature (as in, actual ink on actual paper from an actual pen). This doesn't get around those transactions unfortunately.