I love my fitbit, but don't buy one! As of October 2011, this model is being replaced by a new model, the Fitbit Ultra, for the same price as the original one.
The Fitbit ultra adds a few features, like an altimeter. But it includes all of the features of the original model, so everything I wrote in my review applies to the new ultra model.
I got my fitbit in early 2010 and have been using it on and off (mostly on) since then. I am very happy with it and highly recommend it. It's an easy and fun way to track your activity levels and estimate calories burned, and it should appeal to fellow gadget lovers.
The device weighs under one-half ounce and clips securely to clothing. I'm paranoid about losing stuff (justifiably so), so I keep mine in a little USB drive holder with a clip.
It has an accelerometer chip, like most smart phones, iphones and other gadgets, which can measure its movement in three dimensions.
At any time during the day, you can push the one little button on the fitbit to display an estimate of calories burned, steps taken, miles walked, and a graphic of a flower that grows more petals as you're more active.
When you're near your computer, plug the included fitbit receiver into a USB port, and the receiver will wirelessly collect data from your fitbit tracker, then send the data to the fitbit website. You use this website for most of your tracking and analysis. (The only software you load on your computer is a driver/client for the receiver that plugs into your USB port.)
I find fitbit very accurate for estimating my walking (number of steps and total distance), calories burned, and activity and intensity levels. While these are just estimates, they're pretty good estimates, and they take into account my age, height, and weight. As the data accumulates over time, you can identify trends and measure progress in your fitness regime.
Some people complain that the fitbit system makes you use its website rather than providing software for your computer. I actually prefer it this way. For one, the website is extremely well designed and easy to use. Second, you don't have to worry about using up your hard drive space (an issue if you use a netbook, ipad, macbook air, or older laptop), and you don't have to worry about downloading updates when the developers issue bug fixes or add new features - these just show up at the website without requiring any effort from the user. Third, this way keeps the cost down. Fourth, you can access the fitbit website from any computer with an internet connection.
The website shows graphs of your activity levels and calories burned throughout the day. At this website, you can also enter specific activities you do (like bicycling, working out at the gym, etc). You have to enter these manually, but it is very easy to do so and it's very useful to have a record over time of the specific activities you do.
The website also lets you track your eating. I didn't start using this feature until just recently - I thought it would be too much of a bother. However, I see now that the website makes it very easy. For instance, as you start typing a food (whether from the grocery store or a chain restaurant), it'll suggest a bunch of near-matches from its (extensive) database. You can click on one of these near matches, and it'll fill in all the nutritional info. I force myself to be very honest and complete, even when I break down and gorge on unhealthy comfort food. Keeping a record makes me aware of what and how much I eat, and keeps me honest. And I'm less likely to order a desert after dinner if my log shows that I've already eaten a few deserts this week.
You can also wear the fitbit tracker when you sleep (you slip it in a cloth wristband that comes in the fitbit package). In the morning, sync the data from the device to the fitbit website, and it will keep a record of when you went to bed, how long it took to fall asleep, how often you woke up during the night, and what time you woke up. It tells you your "sleep efficiency," which I think is the time spent actually sleeping as a percentage of time spent in bed. I do not find the sleep data as accurate as the other data fitbit measures (like walking and activity level while awake). For example, it'll say you took 8 minutes to fall asleep when you know that you really took at least 20 minutes: if you lay still enough, fitbit thinks you're asleep. Yet, I find the sleep data interesting and useful enough to keep wearing my fitbit to bed every night: with virtually no effort on my part - fitbit keeps a record over the long term of how much sleep I'm getting and the quality of that sleep, even if the measurements are somewhat noisy. This allows me to identify trends, and if I get sick or feel tired during the day, I can get a sense of whether it might be due to not getting enough quality sleep.
The fitbit is not perfect and cannot do everything. It cannot tell when you're bicycling, or how hard you're working when you're bicycling. While a little sweat or rain shouldn't hurt it, it's not waterproof and shouldn't be worn when swimming.
But fitbit doesn't promise to do everything. It promises to do SOME things (like estimate how much you walk, how many calories you burn, record your activity level at each time of day and let you identify trends). And it does what it promises very very well, in my experience. The device and the fitbit website are well-designed and easy and fun to use, and fitbit completely lives up to my expectations.
At this time (October 2011), fitbit has tons of very positive user reviews. In its early days, there were a fair number of negative reviews, though, but it seemed to me that most people who were disappointed in the device had unrealistic or misinformed expectations when they bought it. My suggestion to you before you order is: visit the fitbit website, read about what it can do, read the FAQ, and maybe google "fitbit review" and look for some reputable reviews (the WSJ has a good one). Then you will get a realistic sense of what the device can and cannot do, so you can make an informed decision. If you like what you see on the website, I'm pretty confident you will be satisfied like me.
I love my fitbit. If I lost it, I'd replace it immediately. In fact, now that there's a newer version for the same price as the original one, the odds of me losing it seem to have increased a bit. ;-D