I love my FitBit Zip and have been wearing it every waking hour for about 10 months now. It is indeed very motivational. Like Max said, I find myself going out of my way just to "get more steps." I also like that it integrates well with MyFitnessPal, which I use to track my exercise & food intake. That app is clever enough to subtract out the calories burned when you wear your FitBit while exercising, so there is no double counting.
I agree that consistency is more important than absolute accuracy. I always wear mine on my hip, and the few times I have checked it against a count of my actual steps, it has been within 1 step in 100. I'm perfectly happy with a 1% error.
For the ultimate gadget freak who is into monitoring diet & exercise, FitBit also makes a scale that has built-in WiFi, so your weight is automatically logged to your FitBit account. Of course, the scale is also clever enough to know whether it is me or my wife that has just stepped on it :)
The bottom line is that in each case (Fitbit clipped to shirt pocket, short center, pants side picket, and pants center), there was some amount of error in the step count. I found this to be a little disquieting. I'm a bit of an obsessive compulsive when it comes to things like this. You can call me old-fashioned if you will, but I do like my numbers to add up to the correct value.
I don't think you should monitor yor blood pressure then. My cardiologist tells me that he hates engineers as patients because they demand a level of accuracy that medicine cannot provide. My home blood pressure monitor takes 3 sequential readings separated by 30 second intervals. The cuff doesn't move. I am motionless. Yet the blood pressure can vary by 30mmHg across the 3 readings which it then averages. I can get a systolic reading varying from 150 to 120mmHg- from dangerous to normal. When am I supposed to worry (in increase by pressure even further)?
I went to the FitBit website and decided that the FitBit Flex Wireless looks pretty cool because it's on a wrist band, tracks sleep quality, has a silent wake alarm and blinky lights. I know my sleep is lousy though, so I didn't get one, but I did notice that the FitBits are compatible with the RunKeeper website. I've recently (and independently) started using RunKeeper with my phone to track my exercise - mostly walking at the moment.
I expect that Truthtable would be concerned about the limitations of GPS precision when walking, and I do appreciate the potential for that concern. However, I've empirically determined that it works well, truthfully.
As far as any concerns about the FitBit go - I don't own one. Regardless, I do understand a thing or two about precision. There is so much variability in daily activity, that being concerned about even 10 or 20% error is really akin to measuring with a micrometer, marking with chalk and cutting with an axe. Given that, I'd have to truthfully say that Max's analysis of the FitBit is very helpful and totally, truthfully, appropriate.
What's more important is consistency. Try to position and measure the same way all the time as much as possible. Then you can compare relative to yourself. But, what? You say? You won't be able to compare yourself to anyone else?. Sorry, but, truthfully, you can't do that anyway with much precision no matter what the tool.