The efficiency is a thing they calculate based on the amount of time you were in bed and the amount of time it appears you were actually asleep.

For example, if I go to bed at 11 but it takes 20 minutes to stop moving, it will record that and tell me. Then, when I rustle during light sleep, etc., it notices that as well. In the end it will show both the amount of time you were in bed as well as the time actually asleep. The efficiency is just the percentage of total you were out.

The FitBits are $99 (they were/are having a sale as part of their integration with Lose It for $89), but to me they are a great value. I've really been impressed with the completeness, accuracy, and ease-of-use of the actual device. While some things could be easier on the website, it really does most anything you'd want and takes virtually 0 effort on your part to make it happen.

Guess you could say I'm a fan.