I just posted this in another thread, so I'll copy and paste it here:

I have a Fitbit One and a heart rate monitor. They do very different things.

The HRM monitors your heart rate during exercise and calculates the calories you burned during your workout -- only during your workout. (Note that it is not considered to be accurate for weight lifting, yoga and other non-cardio type of activities). Most people buy these for the calorie burn. I'm an odd ball. I got mine to monitor my heart rate. :-)

The Fitbit One uses a formula (based on your age & weight) to estimate how many calories you burn all day. This base range can be achieved by wearing it or leaving it on your dresser. In addition, it estimates how many additional calories you burn by using a device called an accelerometer that measures your body movement. It counts your steps, knows if you've been walking slowly or quickly and is supposed to know if you've gone up a flight of stairs (but I've found the stair thing to be somewhat inaccurate).

I don't think there is any wearable device that can exactly measure our calorie burn, our sleep, flights of stairs, etc. But in MY case -- someone whose exercise consists mainly of walking 7,000 steps a day -- I've found the Fitbit One to be a really good estimator. Even better, it's a wonderful motivator!

Some people say the Bodymedia is more accurate because it takes into account body temp, skin moisture, etc. But I am glad that I did not go with the Bodymedia. Wearing it on my upper arm all day would be a problem.

The Fitbit is small, discreet. The biggest complaint I hear is that people lose them. I keep mine clipped to the inside of waistband of my panties. That way, if it falls out of its little holder then it can't fall to the ground. I think this also minimizes the chance that it will end up going through the wash after a quick clothing change.

Also, the Bodymedia charges for its website. The Fitbit is free. Without the Bodymedia website or an extra watch-like purchase, you can't read the day's calorie burn. With the Fitbit One, everything is on the face of the tracker at the click of a button so you can check your progress throughout the day. (note: it does reset itself at midnight)

Just know that the Fitbit does not sync with all phones. If your phone is not on the list, your Fitbit won't get to the Fitbit site or MFP until you get home to your computer. Personally, I don't find this to be a problem since I already know how many calories I want to eat and do not base it off my Fitbit burn.

fitbit is coming out with a new wrist band one.

Yes, I *think* it is supposed to be released the middle of May. Might be hard to find one in the first couple of weeks because a lot of people have already pre-ordered. I noticed that Fitbit and Amazon pre-orders, both show as "sold out".

Do note that it does not count flights of stairs ... which I don't find to be a big loss as I'm rarely in an area where there are stairs, and yet my Fitbit has credited me with several flights.

Since there is no visible data on the Flex (like the Bodymedia), you might want to make sure that your cell phone is compatible, otherwise you won't be able to check your steps, burn, etc. until you are able to sync to your computer:
http://www.fitbit.com/devices

I have the older Samsung Galaxy S II ... which is not compatible. I can download the app and see the data that has been previously synced, but the bluetooth on my Fitbit is does not talk to the bluetooth on my phone, therefore there is no syncing, no updating of information.

I think the Flex looks really cool for the younger generation, especially since you can also buy different color wristbands ... but it's just not my style. :-)