I have a Fitbit Force and previously had a Fitbit One. DH has a Fitbit Flex.
They all have their advantages/disadvantages. I chose the Fitbit over competitors because my research indicated that it was the most accurate, it has a large group of users and I liked how they show their data. Also the Fitbit is a set price with an option for more data under a premium plan. I went for the premium plan (I think it was about $50 a year), while DH didn't.
Anyway thoughts on the different ones I've had and on DH's Flex:
1. The One is very nice. I kept it in my pocket. Some people clip it on their belt. I don't wear a belt and I would be afraid of losing it so I kept it in my pocket. It can measure both your motion back and forth and will measure flights of stairs. So when I walk outside in our neighborhood which is hilly it will tell me how many flights of stairs it is equivalent to. (Note it doesn't recognize incline on a treadmill).
I liked the One because it had a nice display on it so when I was out walking I could click the button and see how many steps I had taken, how many calories I had burned, the time, my distance, my activity level and so on.
The main disadvantage with the one is the fact you carry it in the pocket. So I was always worried about losing it (I didn't, but it worried me). Also, I had to wear something with a pocket. Not all my clothes have pockets (probably less of an issue for a man than a woman). Also, I had to remember to transfer it to whatever clothes I was wearing. It was easy to forget it when I changed clothes.
Also, to monitor sleep (some people don't bother with this) I had to put the one in a large band and wear it around my arm. This never felt really comfortable although I did get fairly used to it. The band did sometimes come off during the night (it was attached by velcro only)
2. A couple of months after I got my One, DH got a Fitbit Flex when it came out. One advantage of it over the One is that it is more water resistant so he can wear it in the shower. The Flex is worn on the wrist.
It doesn't have the altitude function so can't count flights of stairs.
The nice thing about it is that is worn around the wrist so isn't tied to what your are wearing. You largely put it on and forget about it (unless going swimming).
One nice feature of it, is that you can order bands of different colors. DH has a black band and an orange band.
However, I was never tempted to get the Flex. The reason? It didn't have the display of the One. Basically it had a few lights to tell you how close you are to your step goal, but nothing to tell you exactly how many steps you've walked or calories you've burned or the time. DH would have preferred the display but he was sort of OK with just checking on his computer when he would get home. But for me, the lack of a display was a dealbreaker.
3. About 6 months after the Flex came out, they came out with the Fitbit Force which I immediately ordered. It is sort of the marriage of the One and the Flex. You wear it on the wrist like the Flex. However, the Force is wider than the Flex and has room for a display. You press a button and see the time, steps, distance, calories burned, etc. Oh - both the Flex and Force can be programmed to act as an alarm in the morning (I don't use it for that but it can be done - have to program it on the computer).
Also, the Force does do altitude so it counts flights of stairs.
I really like the Force and do prefer it to the One. Having the display is very important to me. Also the Force will sync with my iphone automatically when in close proximity (it only automatically syncs with a very small number of Android phones). It is coming out with a software update where it will vibrate and show the phone number of someone calling you on your iphone ...basically it will act like caller ID (this is optional to use it this way ...it isn't required). Since I have an iPhone I like this. Obviously not a feature if you don't have an iPhone.
It only comes in 2 colors - black and slate - and bands aren't interchangeable. I have the black. I will say that for a woman it might look a little odd to wear this while wearing a formal evening gown. I don't ever do that, though, so it isn't an issue for me.
Because it is worn on the wrist, it doesn't count steps if you hold your wrist still pushing a shopping cart. (This is true for the Flex as well). DH and I have gotten good at pushing the cart one-handed so our steps get counted.
It can't be worn in the shower like the Flex.
Note that for all the accelerometers, they don't do really well counting no step based activity. When I use, for example, my Concept 2 I need to go online and "name" that activity for the Fitbit to know what it is (this is easy to do). Also, if I do weight lifting it will show more calories burned if I go in and name the activity. This isn't specific to Fitbit ... all devices like this are similar.