My journey with personal monitoring devices/pedometers started with Nike Fuelband. I really liked the ability to track steps and also act as a cool clock/watch. After about a year I moved on to the Pebble watch. The pebble was a giant leap forward in having ability to connect to my iphone and also act as a watch. Unfortunately the constant connectivity issues plagued it and I returned it. I decided to give the Flex a try even though its not a watch/clock.
Hardware itself
The Fitbit Flex does have visual feedback in the form of small white lights: They blink as you're charging, they have a special blink if you put it into sleep mode. The fuelband has a very bright and easy to see LED bar.
I find myself fumbling a bit with the clip mechanism of the Flex like I do when I have to button my own shirt cuffs. After it was clipped though, it feels secure and I am not one to put on and remove devices like this. I want to set it and forget it which this does very well.
Fit & Feel
The Fitbit Flex feels very comfortable on the wrist. The pictures don't do it justice. For me personally this is the one defining factor that really makes the Flex shine compared to Pebble and Fuelband. It feels super light weight, doesn't grab, and doesn't look nearly as big/thick as the pictures make it look.
When typing on at a keyboard, I prefer the feel of the Flex. The rubber is flat and thin so it adds very little against the edge of a keyboard.
The Flex does have replaceable bracelets and colors, which means the device is more convertible and flexible going forward. The fuelband wins on the look with the LED clock. The pebble was nice in that you could customize the face to whatever you could find to download.
I wear mine in the shower without issues.
Battery Life & Charging
The battery charging is my biggest gripe with the FLEX. You have to remove the little pebble from the band and plug into port on computer (dont recommend plugging into usb wall outlet). This means you would need to have computer hooked up with power going to this gadget. It does charge fast and mine would easily last 5-6 days if I dont have bluetooth constantly connected but rather sync once a day.
Syncing:
The Flex syncs wirelessly with Bluetooth 4.0 via the smartphone or via an adapter to a computer. For those without a smart phone or a locked down phone that work doesn't allow you to install apps, this could be a deal maker for you. It's not a constant sync but initiates as soon as you open the app on the smartphone. I haven't noticed any significant battery life loss. Not all phones are supported with Bluetooth 4.0-- most phones on the market don't yet have Bluetooth 4.0. You'll want to check Fitbit's website to see if your phone is supported. On the upside, the Flex does come with a Bluetooth adapter you can plug into a PC or Mac to sync wirelessly via the computer. It really adds extra work and decreases the usefulness of wireless sync, but it's do-able.
Software:
The software, to me, is the major differentiating factor between the two devices. The hardware looks entirely different, and there are some small differences in what the hardware can do, but the delivery through the app is what makes them clearly meant for different targets.
The Fitbit software is the same software in use for all current Fibit devices. As soon as you launch it, a sync begins and your latest data will be presented. While the UP presents more current data on the "HOME" page, Fitbit has other tabs where you can explore the data they present. UPDATE: I discovered that the Fitbit software does do single activity trending if you turn the phone sideways: Hours/days/weeks, etc. It doesn't compare it to another statistic, but better than nothing.
Both devices sync your data to a website. The Flex presents a more insightful view of your data on the website.
I am going to keep this until Apple releases their smartwatch
