From reading reviews, battery life seems heavily dependent on what you use it for. One user reported 16 hours of battery life for just reading plain text, with the WiFi turned off -- that might be the max? Others have complained of running out of battery before they finish watching a movie. I haven't tried watching any video yet. But an overnight charge seems to do fine for several hours of book reading + occasional checking of e-mail, which is what I used it for today.
I'm really enjoying the multifunctionality of it; it's nice to be able to do a quick e-mail check and then go back to my book. And it's small and light enough to be a real improvement over dragging my MacBook into campus with me; I think it'll do everything I need on campus, if I store my course documents in the Cloud so I can still print them on campus. Neat.