Lots of quick-release iPhone mounts for bikes out there. At the same time, constantly squinting at a watch is distracting. I wouldn't be inclined to use it for cycling, it's definitely not useful for water sports, and many runners will also object to the need to carry a large(r) iPhone somewhere on their person.

So if it's a sports/fitness accessory, it's primarily for use in the gym.

Really, this feels more like the Newton of smart watches, rather than the iPhone 1 of smart watches. A good start that will become indispensable to a very small number of people.