I'd actually say this device has already existed, or has in part, it's called the nano with a wrist strap, which is a bit erm, uncromulent, they may have fixed that with the UI.
To me the 'watch' has two main functions, one is notification, the other, fitness tracking. Notification, you want a better battery life, such as that offered by the pebble. Fitness tracking, it on paper, lacks many things offered by much, much cheaper devices. Notably the poor battery life and lack of expansion support for say cyclists that you'd find on Garmin.
I'm not slagging people off, calling them iSheep, it's just anything it does is currently better done by established products on the market. The price point is extravogant considering what is offered. Plus it's Apple, you know the early adopters aren't going to be well treated (No gps/3g, no Front Facing Camera etc)