The Zip works okay, but it underestimates calories burned. It takes quite a few days to sync with your steps, and you must wear it in the same place each time for it to be "more accurate"... I wore mine while exercising, and it didn't calculate my steps taken/burns for nearly 30 minutes. It took days to calculate closer to the actual time I worked out, and still (after a few weeks) is off by 10 minutes. It would be much nicer if it was more accurate. All in all, it is fine if you want to see variations in activity.
Love my fitbit zip! Great battery life, easy to use and understand and accurate. I've tried other fitness tracking devices and this is my favorite one so far! A great deal for the price!