I've used a Polar FA20 for about 4 months. I find it a great stimulus to walk every day for at least a minimum distance. I know I wouldn't do this otherwise. But the readings are very inaccurate.
Pros
It doesn't start measuring until you've been walking for about 10 - 15 seconds - so moving about round the house or office is not counted, which is a good thing. (It also carries on working for about that time after you stop walking.)
You can leave it on all day and it will give you the total distance you have walked and the total time spent walking, not counting periods of sitting or pottering around. Alternatively, you can measure the distance for each walk or run individually.
I originally assumed it had to be on your wrist or on a moving limb to work - but this isn't the case. Mine works if its in my shirt pocket as I walk. One day I had to carry a large potted plant for about 300 yards, which required me to have both arms clasped round the pot - so no arm swinging - but the FA20 still measured the distance.
It stores 2 months worth of daily readings.
Cons
Its inaccurate. I note the reading over a fixed part of my daily walk and it regularly varies between 0.9 miles, 1 mile and 1.1 miles, which is more variation than I'd like to see. More seriously, the actual distance of this part of the walk measured using two different online maps is 1.35 miles. This means that it is underestimating the actual distance walked by between 19% and 50% - a huge discrepancy.
Polar sell a separate device ('Flowlink') for keeping track of your results online but the software is not presently Mac compatible.
The FA20 allows you to view a series of screens (current time, current activity state, calories, distance, time active, active steps taken) but doesn't give you the option of suppressing those that you don't want to see. So you may have to push the button repeatedly to find the screen that you do want.
It measures forward motion, punctuated by steps. It won't register other kinds of exercise such as running on the spot. Reviews elsewhere say it doesn't measure cycling or weight lifting.
Conclusion
It is still useful as a stimulus to exercise, but you have to accept that its measuring your distance in `Polar miles' no real miles. I try and walk a minimum of 2 `Polar miles' a day, accepting that the actual distance walked will be between 2.4 and 3 miles. At least its not overestimating my distance. But I expect greater accuracy than this for the price.