I have the Ultra - precursor to the One and I love it.
Tips . . . for me I try to watch the plant - it is an average of your activity over the last three hours. So right now I'm trying to keep at 3 leaves; if I look at its at 2, I go for a walk. You will see on their website that it calculates up what percentage of your day you are sedentary. I couldn't believe how high my number was! So now I'm trying to reduce that.
For the person asking the difference, FitBit is the company and there are 3 models I think - the zip, the One and, newly, the Flex. Each have a little different features.