Fitbit is no stranger to the activity tracking world with several options under its belt, claiming to have one to suit everyone. 

The Fitbit Flex is described by the company as "the stylish wristband that fits your lifestyle" and it will do all sorts - from tracking your steps, distance travelled and calories burned, through to monitor how you sleep.

We have been living with the Flex for a couple of weeks to see how well it fits into our lifestyle and whether its simple design does enough to be one of our favourite fitness trackers, rather than just another rubber band.

The Fitbit Flex is a silicone wristband that features minimal detailing and a sportier design that some other activity trackers. It isn't quite as exciting in terms of design as the likes of the Withings Activité or Jawbone Up24, but that's not to say it doesn't have its own merits in the looks department. It's also significantly cheaper than the former.

The 13.99mm wide band, which is near enough completely plain in its finish, sits roughly 10mm off the top of your wrist at its thickest point, slimming down to a couple of millimetres where it fastens.

The only really distinguishing feature of the Fitbit Flex is the 5mm plastic band which illuminates LED lights to represent your progress. There's also a very small Fitbit logo next to the removable grey fastening on the underside of the device.

Underneath the band itself the Fitbit tracker is placed into a small opening. This pocket remains open so you'll see the tracker if you take the band off, making it very easy to get the tracker in and out - but you'll need to make sure you put it in the correct way round to see the LED light display at the front.

There are three small connection points at the opposite end to the LED lights used for recharging. The tracker slots into an accompanying adapter, which is charged via USB, and we got around seven days use before we had to use it. So battery life is strong with this one.

When it comes to fastening the Fitbit Flex, a similar two-pin system common with many activity trackers is used. We haven't been particularly fond of this two-pin clasp in the past, but as the Flex uses slightly more rectangular pins it's great. It takes some pressure to fasten but this means the Flex feels completely secure and comfortable when we were wearing it, even at the gym.

Two band sizes are included in the box, offering seven holes on the small and nine on the larger one, for a snug fit.

The Fitbit Flex is simple, fuss-free and subtle enough to not stick out like a sore thumb, unless you have opted for one of the brighter colours. We had the orange model but there is everything from lime green and fluorescent pink, to black or slate, with a total of 10 colours available. We did find our model lost its vibrancy and picked up quite a bit of dirt throughout the day but as it's water-resistant, it returned to near enough new after a shower.

We found the design a little chunky and as the Flex doesn't tell you the time, like the Sony SmartBand Talk does, it meant we were having to wear two quite prominent devices on our wrist at all times. Fitbit says the Flex is a 24-hour device - and while it is certainly comfortable enough to wear all day and night, it isn't good looking enough to wear on a night out in our view, unless covered by a jumper or shirt.

The Fitbit Flex is about tracking your movement in the background. It syncs to your smartphone - whether iOS, Android or Windows Phone - via Bluetooth 4.0, but won't deliver any form of notification other than a vibrating and flashing on the completion of your daily goal.

The five LED lights replace a would-be digital display, which light-up when you tap the product rather than always being on. Each light represents 20 per cent of the goal you have set, whether that is a certain number of steps, specific distance or number of calories burned.

Like the Withings Activité, you won't be able to see the exact data until you open the accompanying app, but you will be able to make an estimate based on the goal you have set and the number of LEDs that light up.

Each of the available goals can be altered through the Fitbit app or on the online dashboard so you won't be restricted to the default 10,000 steps with the Fitbit Flex, and we like that you can decide your main goal from the three options. Withings doesn't allow you to change the 10,000 step goal, for example, nor can you choose to have your goal as burning 2,000 calories, so this feature is definitely an attribute of the Flex.  

Through the Fitbit app or online, you'll be able to see your step count, distance travelled, calories burned and active minutes, plus if you link it up with compatible apps, such as MyFitnessPal, you'll also be able to see the number of calories you have consumed and how many remain within a daily allowance.

We found the step tracking was a little overgenerous in comparison to Withings, but it's difficult to say which is the more accurate of the two. Our Fitbit Flex consistently gave us around 500 to 1,000 steps more than the Withings Activité we had on our other wrist during testing, which is quite a significant difference. Even if the Withings was strict with step counting, we would think the Fitbit counts subtler motion.

Distance travelled seemed a little generous too, with the Fitbit Flex logging 5.33km for a 5km run and calories burned was also around 60 more than we expected based on the treadmill estimate. That said, the Flex knows more about us - height, weight, age, and so forth - so it should be more accurate. It's also worth bearing in mind that both the treadmill and the Flex need some form of calibration, so either could be the more accurate.

There is no altimeter on the Fitbit Flex so it can't measure elevation, and the impact of this, but it can be used with MobileRun to track and compare runs, walks and hikes.

Activity tracker by day, sleep tracker by night, the Fitbit Flex also monitors your sleep and can even wake you via a silent vibration-only alarm.

It can log the amount of time you were asleep, how many times and for how long you were awake during the night, as well as how many times and the total length of time you were restless for.

However, our first bugbear with this feature is that it requires you to tell it when you plan to sleep, rather than doing it automatically. You have to add a new log and press "Begin Sleep Now", which you can then edit in the morning if you don't sleep straight away, or fall asleep before you start a new log. To find out how long it took to fall sleep you have to open the graph for that night, tap on it to bring up another graph and then move your finger along to work out the time - which is a lot more effort than other sleep trackers.

It is good that you can still get a sleep pattern if you forget to set the log as the device will pull in the data from that time during the night, but you have to remember roughly when you went to sleep, which isn't always easy. The Flex also doesn't provide you with light and deep sleep information like the Withings Activité does, which we missed, but it did seem to be more accurate in pinpointing times when we woke up in the middle of the night.

As mentioned previously, the Fitbit app is available on iOS, Android and Windows Phone devices, making the Flex a useful activity tracker for the majority. The app is simple and pleasant to look at, delivering an easy to use platform.

Syncing between app and device takes place automatically without issues, and the Flex also comes with a Bluetooth dongle that sits in a USB slot to allow for wireless syncing to your computer when you are within 15-feet too.

All the data - and there's a lot - sits in the app's dashboard. From calories, steps and distance, to calories, how much water you have drunk that day, and the number of active minutes achieved - there's plenty of detail.

Each section has its own progress bar so you can see how well you're doing with a quick glance and they are also colour coded from red to green to show how close to goal completion you are. More detailed information and graphs can also be accessed by clicking on the respective section and you can also remove sections if you would rather not see them on the dashboard.

The Fitbit app is also where you can change certain settings, ranging from the main daily goal, to target weight, through to details such as how often you want the Flex to sync with your smartphone, which wrist it's worn on, sleep sensitivity, or which day you start the week.

For those who need an extra bit of motivation, you can add friends and family on Fitbit.com, where you can compare your stats and share your progress. The only problem with this is that you need members of your family or friends to have a Fitbit device - and as there are so many activity trackers available now, this might not always be the case. You can still earn badges for daily, weekly and lifetime achievements though, so even without Fitbit friends, we found it pretty easy to stay motivated.

Overall the app is brilliant and works flawlessly, it's a real strength of the Fitbit that sees a good product another step closer to great. We might not like the design of the Flex as much as other activity trackers on the market, but the  app is certainly something worth shouting about.

Verdict

The Fitbit Flex is a comfortable, simple-looking activity tracker that might resemble a chunky, colourful rubber band, but brings a lot more brains to the party.

Yes, we would have liked the design to be a little slimmer and more rounded, but on the smart front it's hard to criticise the Flex and its app platform. The ability to choose and adjust your main goal from steps, distance and calories is a great feature, as is the ability to track sleep and more. However, we do think the step and distance tracking are a little over-generous in their calculation. 

With week-long battery life thanks to a simple LED light arrangement, you can wear the Flex and more or less forget about it if you want. It slotted into our lifestyle and we haven't taken it off many times since, only for the occasional smart party.

Overall the Fitbit Flex is a great little activity tracker that runs on a fantastic platform. We weren't totally sold on its design, but as a colourful, comfortable and clever fitness tracker it's hard to complain. Especially with an app platform as good as this.