A couple of months ago DispalyMate's Raymond Soneira posted a technical review of Apple's Watch Flexible OLED display. The display was found out to be excellent, and Raymond said that "there is absolutely no question that for smart watches, OLED is the only way to go for a great display".
The Watch in DisplayMate's edition used a Sapphire cover, while the cheaper Watch editions use strengthened glass (same one used in the iPhones and iPads). DisplayMate said that sapphire's advantage is that it's virtually scratch-proof, but it also reflects about twice as much ambient light as glass. In a new post, DisplayMate compares two Watch editions (glass vs. sapphire). As you can see above, the different under ambient light is quite dramatic.
The conclusion is before sapphire can be used in more mobile displays (for example in mobile phones), the sapphire industry will need to "modify the optical properties of sapphire without significantly affecting its scratch resistance and other mechanical properties".