WHAT CAN TRIUMPH OVER A GOOD UX/UI?
I was excited to hear that ClassPass is now a unicorn. Congratulations to the team! I’ve been upset about the UX/UI design of their mobile and web platforms over a year for several reasons including: 1. the design in the mobile app is not consistent with the flow and design of the website 2. the geolocation feature is sometimes not accurate 3. finding classes is not a fluid experience.
However, I believe that Class Pass is a great example of product/market fit. Where their solution is so desirable that users like me continue to use their service regardless of my frustration with the UX/UI.

DESIRABILITY SHOULD BE YOUR MOST IMPORTANT METRIC
Another example is Craigslist! Do you have others in mind?
Photo Credits: ClassPass