The first day I had it, I woke up around 8:00 AM and got to about 6:00 PM when it ran out of juice. This was with heavy use, screen always on and brightness set to about 60%.
I changed the settings so that the screen is not always on and brightness is now set to 40%. This made a huge difference. Today, for example, I've already had it off the charger for 8 hours and it's still at 79% battery. I'm at work, and it's getting used as it normally would be.
The screen being set to always on will obviously burn through battery at a fairly quick rate, especially if the brightness is set high. It's worth noting that letting the screen turn off automatically does not impede its usefulness. The screen comes on automatically when your turn your writes to look at the device, and when you touch the screen.
Undocked at 8 AM with 100% charged, used till mid night with heavy usage with 32% left when got to bed. Very impressed with battery performance. I am using default settings (brightness - 4, Always-on screen, showing cards on dimmed screen, alert on watch and phone when connected).