Fitbit Flex review: A most versatile, feature-packed tracker

That said, the initiation process for kicking the Flex into sleep mode takes a little practice. You have to tap the device’s screen six times to tell it to start recording your Z's. Only the right tempo of taps will work -- go too fast or too slow and the Flex won’t register your input correctly. By contrast, the big physical button on the Jawbone Up (which also tracks sleep, steps, and calories) is much easier to manipulate.

One upside to the Flex is its LED screen, which displays how far you are towards your daily step goal. Just hit the screen once with your fingertip and the Flex shows your progress on a scale of one to five. When all five lights illuminate you’ve reached the default daily step quota of 10,000 steps. The device will even perform a happy dance when you achieve your goal, complete with haptic buzzing and all lights flashing in a random pattern.

The mobile app shows lots of ways to view your stats over time. Brian Bennett/CNET

Of course you can tailor your goals to match personal targets, too, but not from within the companion mobile apps (iOS and Android). To tweak your custom settings you need to log on to the Fitbit Web site. The Jawbone Up mobile application allows users to adjust their personal fitness targets for sleep and steps, though it lacks a Web-based UI.

I’ve long believed that the strength of the Fitbit system lies in its powerful online data analysis tools. Both the Fitbit mobile app and Web interface let you sift through your stats in views of days, weeks, months, and a full year. You can also pull from a large food database to log meals and count calorie intake. I especially like the ability to create custom foods, and have often-entered meals stored for fast entry. It makes the tedious task of calorie counting less onerous.

To measure weight in real time, Fitbit’s Aria Body Scale also talks to your Fitbit account and provides concrete data to pit against calories consumed and burned. Also, all of Fitbit’s current trackers, including the Flex ship with Bluetooth adapters to sync collected information wirelessly to PCs and Macs. That’s a trick the Jawbone Up can’t tackle. If you own a Samsung Galaxy S3 or Galaxy Note handset, the Flex will also communicate directly with the Flex via Bluetooth.

Select phones can sync with the Flex over Bluetooth. Brian Bennett/CNET

Fitbit says the Flex has a rated battery life of 5 to 7 days depending on usage (i.e., how much you tap the screen). My experience mirrored these claims and was able to last approximately 5 days without needing to recharge. The Jawbone Up, however, has much longer longevity (about 10 days) since it transfers data through a more efficient (but less convenient) 3.5mm headphone jack.

Like the Fitbit One and Fitbit Zip , the Flex seemed very generous when logging my step counts. Many times a week I exceeded by 10,000-step goal by a few thousand. Sure, I live in New York, which can be very hectic, but somehow I don’t think I walk that much.

Conclusion
If you’re in the market for a personal fitness tracker to measure overall activity (plus how well you sleep), the $99.95 Fitbit Flex is an excellent solution. Sure, Fitbit’s own $99.95 One device does have all of the Flex’s core features and throws in stair tracking for good measure. That said, the One's clip design has a tendency to fall off clothes, or stay clipped to them by accident. It's also cumbersome to wear around the clock. Wristband-style trackers are much better suited for this type of duty.

That narrows the playing field to the pricey $149 Nike FuelBand and the slightly less expensive $129.99 Jawbone Up . While the FuelBand is handsome and sports a real LED screen that can display text (and the time), it only works with iPhones and can’t track sleep. The Jawbone Up, on the other hand, logs sleep and steps, though it lacks a display. The Up also requires a physical connection to sync while the Fuelband uses proprietary (iPhone only) Bluetooth technology. By contrast, the Flex is comfortable to wear and has more features, including Bluetooth syncing (iOS and Android) and access to powerful analytical tools. That adds up to a solid fitness tech buy in my book. Of course, if you’re looking to save a little, the $59.95 Fitbit Zip is also a compelling budget choice.

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