I have the Fitbit one and love it. If you get the Flex let me know how you like it. I think it's great that it measures floors climbed as well as steps. It also lets you know your pace and mph on a walk!
It also has a great program for tracking calories, saves cal. count of foods eaten frequently, even whole meals which makes it easy to input meals and snacks. The program even measures your activity against calories consumed and tells you through the day how many calories you have left to consume to meet your goal. You just have to walk near your computer for your stats to update and now they even have mobile apps. As you can see I love Fitbit. I haven't even told you all it's advantages!