I just signed up for Square Cash and I LOVED the experience. The website is so simple and beautifully done and the app is pure joy to use.
The only usability issue I ran into was not having the option to re-send the email verification link. First time I tried verifying my email address I got an error message saying the link had expired, but there were no option to re-send it. I had to delete the email address and add it over again to have it re-sent.