I love my fitbit one. I have never been so motivated. When I see I'm closing in on another badge, I just have to get it! I've gotten up to 20,000 steps in a day and 100 stairs. I like the graphs and seeing the different ways my activity is tracked (steps, stairs, miles, activity points).
I really like how it automatically takes the calories from the food you've eaten and the calories you've burned throughout the day and tells you how many you have left to eat. If I have the munchies, I go up and down the stairs for 10 or 20 minutes and I can eat a little more. With the static tracking with spark, I'd just eat the food and be done.