My friend Raymond from DisplayMate posted the first review of Apple's Watch display. It's a great article and well worth a read. His main conclusion is that the Flexible OLED display is excellent, and Apple did a very good job in its implementation. In fact Raymond says that "there is absolutely no question that for smart watches, OLED is the only way to go for a great display".
The OLED provides very nice and accurate colors and a pleasing picture quality. It compares very good to the iPhone 6's LCD display. DisplayMate tested a single edition, one with a Sapphire display (more about this below). This is a 42 mm panel (312x390, 302 PPI) - most likely made by LG Display.