Can Fitness trackers help you lose weight? The answer is NO!

Update October 2016

When I wrote this post over 5 years ago, at a time when the fitness tracking market was picking up, I may have passed off for a luddite. Now I feel vindicated. A recent study tracking 470 overweight people over 2 years has demonstrated that fitness trackers are completely inefficient at helping users lose weight.
https://www.theguardian.com/technology/2016/sep/21/fitness-trackers-may-not-aid-weight-loss-study-finds
My advice to you: don't spend hundreds of dollars on a gizmo whose only redeeming value is to make you look cool, use the money for a gym membership instead.My advice: don't spend hundreds of dollars on a gizmo whose only redeeming value is to make you look cool, use the money for a gym membership instead.

As an IT person, I am naturally attracted to the newest high-tech gizmos. So I could not ignore the growing trend in the fitness world consisting in using some kind of electronic monitor to measure one's physical activity and the corresponding number of calories burnt.

GOOD OLE PEDOMETER
The ancestor of all these sophisticated contraptions is the good old pedometer that can still be had today for less than $20. All it monitors is the number of jolts, i.e. steps the wearer takes within a certain period of time. Quite useful indeed for somebody whose sole notable physical activity is walking. But how much better (more effective) are those new activity monitors?

CAN TECH MAKE YOU LOSE WEIGHT?
Most of the modern cousins of the pedometer use several sensors to measure additional parameters like body temperature, heart rate, skin conductivity (level of sweating), speed and distance based on
GPS data etc... with the common objective of measuring the number of calories spent during a particular exercise session or a day.
I occasionally use a Polar heart rate/bike computer combo that does a pretty good job at telling me how many calories I have burnt during my 80-minute bike ride and what limits my heart rate should stay within during my workout. Other devices worth mentioning here are the Nike+Ipod sportkit, 24h fitness' Body Bugg and the tiny Fitbit.
I am partial to my Polar gizmo because it is specifically designed for cyclists, whereas the Nike and FitBit can only be used for running.
For you  runners I would recommend either the "Fitbit" or the "Nike+" products simply because the BodyBugg, even though it is more technically advanced than the other two, is way to bulky to be worn on a daily basis, which in my sense is an absolute no go.

WHAT THE MONITOR VENDORS DON'T TELL YOU
Having said that, is this kind of monitor worth the money and, more important, the daily time investment?

My personal answer is a definite "no". There is a simple reason for why I am not using any of those monitors: whereas they all do a pretty good job of keeping tabs on how many calories you burn while exercising, they suck at telling you how many calories you have taken in during the corresponding period of time.
In order for such devices to be effective in helping you maintain a healthy weight or lose weight for that matter, both set of data must be measured with a high level of reliability on a daily basis. For that to happen with these half-baked gizmos, you need to MANUALLY keep tab on everything you eat.

Personally I can't see myself entering the data for every meal I take into a computer (or a cellphone) on a daily basis. This is just too much grunt work (having said I have kept a steady weight without any external "interference" for the last 25 years, so I may not have the strong motivation necessary to do so).

Moreover even if you do get out of your way to enter the data every day, studies have shown time and again that the vast majority of humans cheat or are negligent when asked to report what they have eaten on a particular day. The same is bound to happen when using a calorie monitor (unless you can afford a personal assistant who will do the grunt work for you on a daily basis).

Absent an accurate (manual) accountancy of what you eat, all these monitors are essentially useless because they won't be capable of telling you what your real daily calorie balance is. And as you know, gaining weight is the same thing as having a positive calorie balance over a long period of time and vice versa.

MOST HUMANS CAN SIMPLY RELY ON THEIR INSTINCT
I personally simply prefer to rely on my ability (which I share with many humans) to instinctively eat EACH DAY according to my level of activity on this same day. If I could not work out at all, I systematically eat fewer calories than on a regular day (I generally eat 3 meals on  that day instead of 4 on a workout day).
I do not need a monitor for that and I am not alone. Most human brains can do the math unconsciously. This is why most humans are NOT overweight. They know how to do this without even thinking about it.

If you still want to use a monitor, because for example you are very overweight and you absolutely need some hand holding, then be ready to invest a not-so-negligible amount of time every day to enter data into your computer or cell phone and try to be honest with yourself (not an easy thing to do as illustrated above).

As for me I will continue using the corresponding time saved to take a nap or spend a few more minutes a day on my trusty bicycle.