Second, the daily/7 day/28 day views have had a real cognitive impact on me, leading to noticeable behavior change. Ya can't AB life, so part of what has happened is probably attributable to improving weather, also, but I had been trying to take multiple walks a day for _months_ -- since before the bad weather set in -- and failing. But for whatever reason, the kind of feedback I get from the FitBit means that I actually go take a walk in the morning, a walk in the afternoon and one in the evening.
Third, having friends actively using FitBit helps also. I specifically asked R. not to taunt me, because I have a nasty competitive streak and I do a lot to keep it under wraps so that I don't make everyone who knows me hate me. Nevertheless, seeing where my (short) list of Fitbit Friends are on the weekly step count is an ongoing motivator.
In my month-ish of using the Flex, I have found the step count to be basically accurate. I don't really pay that much attention to the distance estimates, because I know my stride length is ludicrously variable.
I don't know if these are the effects you would experience, or if you would want them, but I'm really happy about the Flex. It is less intrusive and less prone to falling off than the Omron I wore on my waistband or pocket. And the more detailed feedback, retained for longer (the Omron pedometer remembered 6 or 7 days, IIRC), makes a big difference, even tho I don't really understand why.