I have been using a Fitbit One for almost a month now and i think it great!
Its small enough that it doesnt get in the way, but you do forget about it sometimes. If i get changed i often leave it in a pocket and remember it a while later. I found its best to clip it inside my trouser pocket as far to my side as possible. The button does not seem to get pressed by accident.
The sleep monitor setting is good. interesting results seeing how long it took to get to sleep, how many times awoken and overall length of actual sleep. You have to wear it on the wrist band provided. Its slightly uncomfortable at first but you get used to it quickly.
It logs the number of steps and floors climbed, it logs the calories from this really well i find. it has a clock display and you can set a number of silent vibration alarms via the website.
The website is easy to use. Sign up to create a profile and the fitbit uploads automatically everytime im near my PC. I just have to leave the bluetooth dongle in one of USB ports, Luckily i have many as there are so many bits of technology to charge or connect. I enter my activity, weight and food manually onto my profile too so all the info i need is in one place. The food log is time consuming if you have to add the food manually but i have found that there is usually something already programmed in thats close to what im having. Just make sure the calories are as accurate as possible.
The only negative thing i have found with the Fitbit One is that its another bit of technology with its own type of charger cable. Would it really be that much to ask that all companies use the same one???? So if i lose this i am screwed. Also it says to plug it in to a laptop or PC but non of these charge the thing. It only charges from the car adapter for some reason. Probably due to the power output being different. I dont know the exact reason but i am in the car enough so this is not a problem to plug it in there once a week.
So overall its top notch. Go get yourself one. A very reasonalble price as well at around £60-70 atm.