I ended up getting the Fitbit Force as a Christmas gift. I've been through one charge cycle with it. Here are my observations so far:
The Force is bigger than the Flex. All of the tech is built into the band as one piece. In other words, the tracker cannot be removed from the band like it can be with the Flex. To me that is an advantage - it's a more solidly constructed object that way and it allows me to wear it more loosely on my wrist without fear of the tracker falling out of the back of the band. The clasp is a bitch to secure.
The screen on the Force is nice. It tells me what I need to know in a more straightforward manner than the dots on the screen of the Flex. It can be read easily in all kinds of light. It's customizable to an extent - through the app you can add or remove items for viewing, change the appearance of the clock/watch and put items in a different order. For instance, I got rid of the clock since I wear a proper watch on my other wrist and got rid of the "active minutes" since I think that is a somewhat arbitrary measurement that doesn't provide much insight into the type of activity I do. The only thing I wanted to change with the display that I cannot is getting rid of the silent alarm view. It's a recurring weekday alarm, so I don't need to see it since I already know what time my alarm is set for. For whatever reason, that's the one display-related thing that isn't customizable.
Speaking of the alarm, it has turned out to be the killer feature (for me) that I thought it would be. I have trouble waking up to audible alarms and can often sleep through them. That annoys the shit out of my wife who gets up before I do, and then listens to me sleep right through all of the most grating ringtones on my phone. The Fitbit just buzzes my wrist and wakes me up right away. It has worked 100% of the time so far and has annoyed my wife 0% of the time.
In terms of measurement... I've found it to be pretty good (or good enough for me) so far. Since it's worn on your wrist it does measure steps sometimes when none are taken. It's pretty good at discriminating between general movement and the type of movement found in walking, but some hand motions that approximate that (certain hand gestures, winding a cord) can create false positives. There have been other things I've tried where I thought extra steps might be recorded but when I checked none were. I think if you look closely at small samples you would find some discrepancies but in larger samples things tend to even out and it's fairly accurate overall. It estimates distance in miles as well based on your stride length. For me the default stride length was pretty much dead-on over a span of .55 miles, but you can also enter your stride length if you need to fine tune the accuracy of those estimates.
The Force also measures flights of stairs climbed, It does this with an altimeter that measures slight changes in barometric pressure. When it detects an ascent combined with walking motion over a rise of 10 feet, it records that as a flight of stairs. It has been very accurate on this front so far. I haven't seen it fail to record a climb yet. Sometimes it gives me credit for an extra flight of stairs, but that's usually out in public where a flight of stairs covers a greater ascent than the flight of stairs in my home. For instance, at work I get credit for going up two flights of stairs by walking up to my second floor office. Makes sense to me though.
I have used it to measure sleep. It measures time, restless minutes and awake minutes. Awake minutes aren't very accurate since it seems like you have to get out of bed for it to know that you're awake. I guess that restless minutes are fairly accurate, though I guess by definition I can't know for sure since I'm still asleep at those times, but tossing and turning. The information is interesting to see when it's compiled, but I'm not sure I have much of a use for it.
The app is great. I have used it for months now, long before I decided to get serious about getting one of these devices. To me the Fitbit app was the best app for tracking weight and water intake that I could find. That made it really easy to get the hang of everything right out of the box. The nice thing about a device like this one is that it takes the burden off of my phone for tracking things and takes the burden off me for remembering to track things manually. I just open the app occasionally and everything syncs over Bluetooth.
Battery life is excellent. They advertise 7-10 days, and I got every bit of ten days from the first charge cycle. The USB charging dongle leaves a bit to be desired - and it's a proprietary connection between the dongle and the device. At least with it being USB I can plug it into the AC adapter brick for my phone/tablet and charge it that way.
Unlike the other tools that I had been used to track things over my phone it doesn't require GPS and doesn't eat my phone's battery. Plus, since it's measuring and counting constantly it tracks things that I could never begin to measure before. I'm not a workout fiend - my bum knee keeps me from doing very much beyond walking for exercise. I do walk a lot these days and I have been meticulous about using RunKeeper and other tools to track distance or as a place to manually enter my workouts. I was really only able to measure certain activities though - when I exercised intentionally either on a track, on a treadmill, or outdoors. Since walking is how I exercise and walking is something I do all the time, even when I'm not intentionally exercising, the Fitbit gives me a way to track all of that "hidden" activity. For instance, last week on a day where I went shopping at Best Buy, Lowe's, Home Depot and ran other errands, I actually did more walking than a couple days later where I sat around most of the day watching football and spent an hour on the treadmill trying to make up for my laziness. Before I could reliably track that stuff, I would have thought that the treadmill day was the more active day because that was the day I went out of my way to exercise. It has made me more aware of the little things I can do to be more active throughout the day. It allows me to observe general trends in my levels of activity or inactivity.
Conclusion/TL;DR... If you need a silent alarm, it is excellent. If you are an exercise junkie or do intense non-walking/jogging workouts there probably isn't much benefit to something like this. For the kind of activities I do, my current habits and the habits I'm trying to develop, it's just about perfect and I'd recommend it on that basis.
