Keep in mind this is just a preview and subject to change as we continue to develop the software. For example, the font size is customizable, eg you can select to have emails shown on screen in larger print. The next step is to implement this UI for the watch itself and test it thoroughly on a prototype Pebble.
Pebble, one of my highly anticipated tech gadgets that I (and a million others) kickstartered awhile back, has released a preview of the UI. While watching the video, I was paying more attention to how he was testing the UI than the UI itself, which is early iPod-y and simple. It looks like they are using the old Mac font, which is kind of a neat throwback. Martijn, Pebble's User Interface designer and iOS developer, is using some kind of remote screen which reflects what he's designing, to the Pebble screen. This software will be released with the Pebble, which I'm very excited to hear. Martijn also placed his iPhone on his wrist while testing, as it may look silly, it's a good sign that he's thinking with the product's use in mind. I can't wait for this to come out!