I've just released version 1.0.1 of an Apple Watch app called Battery Monitor, which displays the connected iPhone's battery life on your wrist.
It's pretty simple, but check out the marketing materials and the features below. Unfortunately it was not possible to add notifications due to Apple's API restrictions. Also time based battery life estimates aren't included since they would be highly uncertain.
Version 1.0.1 Features:
- An Apple Stlye animated ring displays remaining iPhone battery life
- Text based overview of the current battery status
- A lightning bolt symbol shows charging/discharging status
- Apple Watch 'Glance' for quick access to the information