Jawbone Up Questions

Recently, there has been some interest in "life-monitoring" devices such as Up by Jawbone. Before I consider getting one, I'm asking these questions:

• What can it measure if I just set it up and put it on, but do not bother with the button and modes? Actually, I'm wondering if it can tell when I'm sleeping or exercising on its own, or do I have to tell it?
• What can it measure if I put it on and use it as instructed, but without other devices? Other than pairing with a smartphone, but not taking pictures or anything like that, what can it do?
• What can it measure if I follow all instructions?

These questions are actually asking how easy is it to use.

Also, what does it really measure? Steps? Energy used? How does it measure calories eaten? What exercises does it detect?