Great question....I was researching these yesterday on Amazon. I want a fitbit or something similar for a little more accuracy than MFP. The synchronized scale look appealing too but a bit pricey.

I have the polar FT4 (http://www.amazon.com/Polar-Heart-Rate-Monitor-Purple/dp/B005M1P85O/ref=sr_1_1?ie=UTF8&qid=1389128159&sr=8-1&keywords=polar+ft4), and I love it. It's really straight forward with setup and care, so I actually started using it the day I got it. I am also the type of person who is more motivated to hit the gym if I have cute clothes and accessories, so the bright pink watch was perfect for me lol.

I just got a fitbit flex on Sunday. So far, so good. It tracks your steps, sleep pattern and syncs to MFP. It has motivated me to move more during the day to get to my total step goal. I got mine at target because of the $10 gift card offer.

I just went through a ton of research and wound up getting me the Fitbit Force last Saturday. Here's my long-winded take:

I can't say I know too much about Polar devices aside from the fact that the company specializes in health monitoring (when I took a rugby exercise class in college years ago, everyone was wearing Polar heart rate monitors.) I wanted something stylish that I could wear 24/7 (rather than just during workouts) so I stayed away from the Polar products.

I also wanted the sleep monitoring functionality (I have moderate sleep apnea, so I use a CPAP machine), and I wanted wireless/automatic syncing, so my ultimate decision lay between the Jawbone Up 24 ($150), the Fitbit Flex ($100) and the Fitbit Force ($130).

I went with the Force for a few reasons:

1) I'm not a big jewelry/adornment fan, and I just wasn't keen on wearing a "bracelet" (which is exactly what the Jawbone is). The Force looks like a digital watch.

2) I chose the Force over the Flex because it really *is* a digital watch. It has a crystal-clear little lit LED display (you have to press to turn on, tho -- saves on battery life.) It even has a stopwatch function to time your workouts, which it remembers and shows separately on the app/website (so you can see just how many steps/cals/miles were in that particular workout.)

The Fitbit Flex is supposed to have most of the functionality of the Force, for $30 less. It's basically only missing a digital display; instead, it uses a little meter that consists of 6 LED lights that show your progress for your daily step goals.

2) The Force also has a built-in altimeter that registers the "floors" climbed (as in flights of stairs). If you're working out in a gym, this probably won't be of much use to you, but most of my exercise takes place walking/hiking on the rolling topography of Central Oregon. Knowing those extra calories burned when going up and down hills is extremely valuable, I think. (I hiked to the top of a tiny volcanic cinder cone that's 1/4 mile from where I live, and my Fitbit registered about 50 flights of stairs -- fairly accurate, considering the butte is a 480-foot ascent, and the 1/4 mile from my house to the trailhead is uphill as well.)

A pleasant surprise with the Force: I thought I would have to regularly fire up the iPad app in order to bluetooth sync, but it turns out they include a little USB transceiver dongle that you leave plugged into your computer. (You download and install a little sync app that runs on the menu bar from their website.) If you're at home/within range, and if your computer is on, the dongle automatically grabs the information from the wrist device at regular intervals (or when you first get into range -- the transceiver seems to know when you were away and it syncs immediately upon getting home) and uploads the info to the Fitbit website, which in turn gets the info to MFP. I still have the option of forcing the sync (if I don't want to wait the 5-10 minutes for the automatic sync), or using the Fitbit app to sync through my iPad's bluetooth (if I'm not at home) which then uploads it to the site.

The distance calculation seems pretty accurate. I've measured both a walking and jogging distance (1.8 and 1.4 miles, respectively) by tracing my route via Google Maps, and Fitbit gets it within 5% of the actual distance. There is, however, a calibration feature where you walk a measured distance on a track and count your steps, use the info to calculate your average stride distance, then plug the data into the Fitbit app/website so that the tracker is more accurate, but I haven't had the time (or the track!) to try it yet...

Sleep tracking is kind of "meh" for me so far (ironic considering that this is the feature that narrowed my search down to the three), but I'm not sure if it's the device, or the fact that I move a LOT while I sleep. I'm bouncing between the "sensitive" and "normal" sleep settings to see which ones seems to reflect my night's sleep the best. But at the very least, it shows my bedtime and wake time, which makes me more conscious of going to bed and waking up at a decent hour.

TLDR: So all in all, I'm pretty happy with it. It's accurate enough for my needs, and having it on 24/7 not only makes for some good, motivational metrics (seeing total calories in vs. calories out is a great gauge on how I'm doing!), and its presence on my wrist is also a reminder for me to stay mindful of my food and my activity. I catch myself checking my steps every so often throughout the day, forcing myself to walk a little bit more to reach my 11,000-step-a-day goal.