Docusign is indeed pretty terrible as an API, but it does have the benefit that users have heard of it so it has a certain institutional credibility - which is usually important if you're developing the sort of app that needs an e-signature process. On that front, all of our clients were very comfortable with Adobe as a secure service provider - it was something we publicized, in fact.
I've played a bit with Hellosign. While I've heard they have a nice API, they don't have the institutional credibility that Docusign and EchoSign have. Also, their pricing was way, way higher than EchoSign's, in my experience. I'd love to learn more about them, though.