I've been impressed by Docusign the few times I've had to use it. And I generally hate 95% of the software I come across. Everything seemed to snap into place, was perfectly intuitive. That stuff is easy to get wrong, as evidenced by all the crap out there.
It's not managed to annoy me, which is quite impressive. I never really considered Docusign anything special for usability, until your comment, but that's because it gets out of your road enough not to notice it.
And to their credit, I've been able to sign documents on Linux via Firefox without hassle. Half the time, some shitty SaaS apps will just sniff my user agent and block me.
And it may be worth noting that the alternative is often to go physically into an office someplace or get various notorizations or Medallion Signatures through your bank.
They do substantially more than the click-to-sign part. ID verification, contract management system, electronic notarization, APIs for automation of the whole thing, mobile apps...