Apple became the world’s leading vendor of wearable bands in the second quarter of 2015, according to Canalys (www.canalys.com) estimates.

With 4.2 million shipments, it easily overtook Fitbit, Xiaomi and all the smart watch vendors, despite the Apple Watch’s significantly higher pricing. Apple has done an excellent job designing its WatchKit SDK [software development kit], which it delivered "quickly and comprehensively," says the research group.

Native WatchKit extensions provide a capable developer platform without sacrificing battery life. But early sales were hindered due to two factors. Firstly, Apple missed the fourth quarter 2014 holiday season. Secondly, supply constraints stalled shipments for the first couple months after launch.

"The Apple Watch is the most sophisticated smart watch to date, and it has proved popular with Apple fans worldwide," says Chris Jones, vice president and principal analyst at Canalys. "But Apple and other vendors still face important challenges to make the smart watch a breakout hit. Improvements in performance, battery life and sensor integration are needed to make future models more attractive, but it is the quality of third-party apps that will determine whether the Apple Watch will be a long-term success."