There is always the question of legality when you mention signatures on a piece of paper and with e-signatures it is the same question, Is it Legal? There are two laws that help to define the legality of e-signatures:

The E-SIGN Act (Electronic Signatures in Global and National Commerce Act) was passed by Congress in June 2000 is a federal statute that establishes the legal equivalency of electronic contracts, electronic signatures and other electronic records with their paper counterparts. It specifies that in the United States, the use of a digital signature is as legally valid as a traditional signature written in ink on paper. The E-Sign Act applies to all types of transactions, whether in interstate or foreign commerce, unless a specific exception applies. The legal strength is due to the robust authentication data captured by online signature software, which provides digital evidence of who signed a document, as well as when, where and how they did it.

The UETA (Uniform Electronic Transactions Act) works in conjunction with the E-SIGN Act to help ensure the validity of electronic contracts. This act was approved by the National Conference of Commissions on Uniform State Laws in 1999.