So, if your wrist movement is ANYTHING other than the usual movement of either running or walking, your step count/mileage is going to be off - often way, way off.
Fitbit markets the Flex as an "activity monitor", implying that it is capable of keeping stats on your physical activities. Well, because of the wrist location issue and the basic design structure of using two stride lengths for all calculations (your walking and running stride lengths), pretty much any activity that is NOT walking or running cannot be accurately monitored: the accelerometer senses whether you are walking or running and multiplies the appropriate stride length by the number of steps sensed (and arrives at a mileage distance). Any activity outside of that limited ability of the Flex, your stats are innacurate. This includes stair-climbing, hiking, tennis, golf, biking, working out, and pretty much every other "activity" that is not walking or running. The Flex is not really an "activity monitor"; it's an expensive pedometer (albeit one with "sleep analysis", which is actually only a record of how much wrist movement you had during the night: i.e., 2 hours of no movement = deep sleep, an hour of sporadic movement = restless sleep).
There are, however, aspects of the Flex that are quite good: it does accurately keep track of your walking and running, it does keep track of your sleeping patterns, and it also wakes you with a silent vibration on your wrist at the time you set to get up in the morning. And once out of bed and into the shower, there's no need to take the Flex off: it's almost completely waterproof (pressurized water being the exception). But the best, perhaps life-enhancing aspect of the Flex is its ability to keep you focused on your exercise goals (which is the main reason why, overall, I like the device and its software - to a point): if you DO consider it only a pedometer with cool software, which is what I consider it to be, you will not be disappointed (though the price may not seem worth it to others).
First, there's the 'onboard' function to keep you aware of your progress towards your daily goals (total distance traveled, number of steps taken, or calories burned): each light on the row of 5 small LED lights on the band represents 20% of your goal and a quick glance and a tap on the band will tell you how you're doing. Secondly, for the actual stats of your progress (or reviewing your daily, weekly or longer stats), you can view them in real time on your phone via the Fitbit App for iPhone and Android, or on your tablet or PC/Mac with the Fitbit 'Dashboard'. This is where you set your daily and longer-term goals, view not only your stats but see them represented in charts and graphs, and log other information such as weight, weight changes, and food consumed. You can go further, but personally I would feel a bit silly setting goals where I would receive 'badges' for reaching them. But that's just me. Overall though, the software is very good and keeps you aware - along with the band's LEDs - of whether or not you are staying on track of exercise goals. But, again, just keep in mind that the wrist placement of the device gives the Flex an inherent flaw as to the accuracy of all this data - UNLESS you are using it only to monitor and analyze walking and running stats, as I do.
If you're like me and the term "highly-motivated" (or "disciplined") and your name rarely appear in the same sentence, the Flex is a great help in that regard: it makes you keenly aware of whether you're succeeding or failing in your exercise regimen, which I find to be a great help in discipline and motivation. The Flex is a good motivator because you do not want to have today's exercise stats look inferior to yesterday's - or this week's less than last week's - or this month's less than last month's: you can graphically SEE all this on a color graph, your success or failure staring you right in the face. And who wants to stare at his or her failure or laziness? We want to see improvement, to see success. And, for me, this is why the Flex is a valuable tool: an ever-present motivator. I personally use it for running and I find that I really push myself to prevent the present week's stats (total mileage, etc.) from failing to outdo last week's. Seeing the charts and color graphs of several months of steady progress, there's a real feeling of accomplishment and a strong motivation to continue. It is this visual presentation of your exercise efforts that sets the Flex (and other Fitbit models) apart, in my opinion.
Is the Flex a miracle device that will accurately detail your every movement, every calorie burned, throughout your day? No, it's an accelerometer/pedometer based on your stride lengths and your wrist movement; there are many activities where it is impossible for such a device to determine steps, etc. Fitbit's marketing of the device was overblown, calling the Flex an "activity monitor/tracker", implying - to many - that all activities could be measured for steps/mileage/calories, which is not the case. Further, Fitbit should not be marketing it as a calorie monitor because an accelerometer alone (which is what the Flex is) cannot possibly count calories burned with any real accuracy.
So, overall, I would give the Flex barely 3 stars, based on the way it's marketed to the general public. It does fail to live up to many buyers' expectations (read the many 1- and 2-star raging reviews). The technology required to do what Fitbit infers the Flex can do - tracking and monitoring most physical activities - is probably a few years away. Someday there may be a Flex with integrated GPS, heart-rate monitor and an accurate altimeter: THEN we can expect steps/mileage/calories counts to be done for many more activities. For now, this Flex ("Flex 1.0") is basically a pedometer (with a few other features) and its accuracy is limited to a narrow range of activity. My personal needs happen to fall into that narrow range: I use it only for running and walking exercise and in analyzing, graphically, my fitness history and progress via the Dashboard, and it's a great motivational tool. For my personal needs, I'd probably have to give it close to 4 stars (but this would be misleading to the average Amazon buyer). Unless your personal needs are as narrow as mine, I would hold off on buying the Flex until improvements are made. You can duplicate this model's limited abilities with a decent pedometer and freeware/shareware to manually input and analyze the stats.
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UPDATE
There have been so many good questions about aspects of the Flex that I didn't cover originally that it's probably best that I expand the review:
LOCATION OF TRACKER: Early questions asked about whether or not you can wear the tracker outside of the band or with the band around your ankle, etc. (earlier Fitbits could be worn anywhere on the torso). It is only accurate on your wrist (or, rather, it is most accurate on your wrist, its overall accuracy pretty questionable).
ACCURACY OVERALL: For usual activities like walking and running, if you see inaccuracies in mileage or number of steps, probably your stride measurements are not like the average person for your height (which is what Fitbit uses for default strides, your height entered when you set up your account). In your account settings, you can change 'Stride Length' and 'Running Stride Length' in order to calibrate the accuracy. Ignore the reviewer who states that you can't calibrate the Flex: you can; all the tracker is doing is counting steps (and multiplying those steps for mileage amounts); since you can adjust your stride lengths, you can calibrate the Flex to your exact needs. The device has a walking step (stride) and a running step and counts them (and converts your steps to mileage). The only way to have accurate mileage is for the Flex to KNOW your two stride distances and to have the wrist tracker sense whether you're walking or running; if you mess with those dynamics (irregular steps, irregular wrist movement), you mess with accuracy, simple as that.
There is an additional tweak to minimize wrist movement inaccuracies: you CAN go into your dashboard settings and change the location of the wristband from 'dominate hand' to 'non-dominate', but I've found that this to be of little use: the Flex is only reliable when walking and running AND having normal wrist activity while doing so. Instances where one wrist is more active than the other are not likely to be in walking and running activities.
WATERPROOF: It is not entirely waterproof. You can wear it in the shower or along the surface of a pool. However, it's not rated to withstand water at depth - water where there's pressure.
STAIRS: Unlike its predecessors like the One, the Flex does not have an altimeter and therefore cannot count stairs climbed. From what I read, Fitbit did away with it because it was inherently inaccurate.
BANDS: The Flex comes with both a small and a large band. The large band goes from about 6 1/4" to 8 1/4" and the small, 5 1/2" to almost 7" and both are fully adjustable between those lengths. Putting on the band is difficult the first week or so - the rubber is a bit stiff - but then it becomes looser and easier to put on.
TREADMILLS: Your 'Running Stride Length' on a treadmill may be shorter than on the road (no forward momentum). See above about tweaking stride lengths.
HEART RATE: No, there is no heart rate monitor.
CALORIE COUNTS: The Flex really should NOT be marketed as a calorie-burning monitor. No device without a heart-rate monitor can accurately determine calorie burning. But when using the Flex to simply analyze a run or a long walk, it can 'ballpark' calories burned fairly well; expecting it to tell you how many calories you burned in the past 24 hours - not so much.
SUBSCRIPTION: For your normal account there is no additional charge for anything but for a 'Premium Account', which gives additional analysis and tips (such as a 'Fitbit Trainer' 12-week plan, more data analysis, a 'peer ranking', etc.), it is $49.95 per year. The Premium doesn't really seem worth it, to me, but then again it's only $4 a month or so for someone who wants to get more from the Flex.
SLEEP MODE: There are two options for sleep mode. One is automatic and does not require you to "tell" the Flex that you are going to sleep, like the Up and and other competing models; however, you do need to log your sleep hours on your dashboard; at the next sync your sleep data will be visible. Alternatively you can fast tap the band when going to bed and do the same when you wake up. Personally, I prefer the dashboard method as I would probably forget quite often to tap the band. The Flex determines amounts of restful vs. wakeful/restless sleep simply by wrist movement (i.e., no movement for 2 hours = restful sleep). The novelty of being able to analyze your sleep patterns soon wears off, at least in my case; the usefulness of the data seems questionable.
ANDROID COMPATIBILITY: You need Bluetooth 4 for real time syncing.
CHARGING: It takes about 2 hours to fully charge using the dedicated USB cord. The battery lasts about a week.
OVERALL OPINION: I still like this thing: it keeps me motivated by being able to compare walking and running stats over time via the software interface. Otherwise, it's no better than a decent pedometer (and double the cost). Remember that this is the 1.0 version of the Flex, so enhanced capabilities may not be too far away. Perhaps the Flex 2.0 will be worth the list price if improvements are made but this 1.0 version may really not be worth it for most people. Read the many 1- and 2-star reviews (over 250 of them) as evidence of this.
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