I have a Fitbit Charge and I have had it for about 6 weeks. I've found that its really good in terms of motivating me to do more exercise and the app that tracks food as against your calorie burn etc has really helped me to make better choices about food (both in terms of what I'm eating and how much of it I should be having). I have lost a stone (14lbs) since I have had it so something is working! It's comfortable to wear and seems to be relatively accurate in terms of tracking steps. It doesn't track other forms of exercise so well - for example I bike - but there is an ability to log other exercise manually in the app. I have been charging it up once a week or so and that seems to be fine. It emails you when its getting really low (plus you can see the amount of battery life left in the app).
Downsides however for me are:
(1) The food lists on the app are very eating out focused (rather than having home cooked food as I tend to) and US centric, so not that applicable to me as I am in the UK. However, I understand that the my fitness pal app is better and can also be linked to the Fitbit Charge (though I haven't tried this).
(2) I have an intermittent glitch with the app (which I have reported to Fitbit and they are looking into) in that sometimes food logs are deleted randomly, which if I don't catch it, could catch me out in terms of how many calories I have left, and I could go over what I am supposed to, and also that its maths goes a bit crazy on adding up the calories for previous days (even though they were fine when I logged them) - I have allegedly eaten 68,000 calories in a day according to the app sometimes!
(3) It's not waterproof so you have to remember to take it off and put it back on again when showering etc.