[tofflos]

I'm not that familiar with TLS client certificates so I'm not qualified to say, but if you consider other developers as your users, then the UX problem remains.

Web developers in general are more familiar with other forms of authentication so unless you have a strong reason for picking TLS client certificates I would suggest picking something else.

In other words: I would be more likely to try out an API if it was based on Basic Authentication. ;-)