As a disclaimer, I have not used any of these devices so the following comparison is purely based on their feature sets. If you have used any of these gadgets, please share your experiences in the comments below.
Let's start with the sensors. All three pretty much use a 3 axis accelerometer to determine movement. The Fitbit also includes an altimeter which measures the altitude of an object so it can tell if you are climbing stairs.
If you're an avid cyclist, you're at a loss because none of these wearable gadgets can tell if you are cycling. This can easily be resolved by linking the device with a mobile phone's GPS to allow you to track your rides.
Also none of these devices are waterproof (although they are water resistant) so don't plan on tracking your surfing or laps sweated in a pool.
Next up: feedback. All three devices offer entirely different feedback loops for the user. The Fitbit has the most sophisticated visual display (an OLED) that offers cheers of encouragement and motivational phrases such as "Walk Me." It also uses a graphical icon of a flower that grows and shrinks depending on your level of activity.
Similar to the Fitbit, the Fuelband uses an ambient progress bar, changing from red to green throughout the day as the user approaches his goal. The progress display although is limited to 20 color LEDs and the main display to an array of 100 white LEDs.
The UP, on the other hand, doesn't have a visual display. Instead, the UP opted for the use of haptic feedback, using a small motor to alert people of their inactivity.
Personally, I like the idea of using haptic feedback vs a visual display but, unfortunately according to the reviews I've read, the motor in the UP has the tendency to stop working altogether.
All three devices are accompanied by a mobile app that graphs and displays the collected data. In addition to tracking activity, the Fitbit and Up track the user's sleep patterns and provide a daily food diary feature.
The Up also has a Smart Alarm feature which will wake up the wearer with a gentle buzz.
The Nike+ Fuelband app takes a slightly different approach and bases the user experience on setting and reaching personal goals. While the Up has sponsored Challenges, they are broad and not catered to an individual's needs.
It is in the social integration and overall user experience where Nike+ Fuelband has a clear advantage. Users can tap into the rich Nike+ online community which has been thriving for years. Nike+ offers the ability to share goals and triumphs on Facebook and Twitter and organize and compete with friends.
The Fitbit and Up also offer the ability to organize in groups and compete with friends but since their user base is quite small, it may be a bit more difficult to find a teammate.
Lastly, I appreciate that the Fitbit, as they say on their site, "plays well with others." In other words, the Fitbit partners with other nutritional and fitness apps making it easy to sync your stats and personalize your fitness strategy.
Once I get ahold of these three devices, I will provide a detailed review of each product and their user experience. Heck, I may even hack into one and customize it to my needs.