Basically, an accelerometer measure how much it is jostled during the day. So if you wear one on your torso, it can measure how active you are. However, it will not measure activities such as biking or weight lifting, since your torso will not move much doing those types of activities.
Davi Pogue's aticle "Getting Fit With 2 Bits of Help" summarizes the pros and cons of the Fitbit and DirectLife accelerometers.
The Fitbit not only tracks your torso motion, but will also track your sleep after you set to that mode of tracking. You can have the Fitbit wirelessly transfer its data to the web via the charging stand and software. This will occur everytime you pass within 15 feet of the charging stand. You have to analyze the data yourself, however. Fitbit doesn't provide analysis or guidance on the data.
The DirectLife device doesn't track your sleep or have wireless data download. You can download its data directly by connecting it to it USB docking cradle/charger that is connected to your computer. DirectLife does employ experts to look over your data, answer your questions and give moral support at a cost of $12.50 a month. This device may be more durable than the Fitbit, according to Pogue. It also cost a little less - $80 versus $100. It is also waterproof.
Charles