[gjhiggins]

You mean UI not UX, the difference will be crucially important to you.

When inviting informed review of an app, it's useful to outline the design brief.

Given the number and nature of the UI issues reported by other HNers, you might be interested in making use of the resources offered by http://www.usability.gov.

Lastly, I'm a little surprised that your prior user tests didn't give you any hint of the degree of failure, you might want to take a closer look at your user testing process.