I have a fitbit flex.
I have very mixed feelings about the flex to be honest. It fits around the wrist, which is great, but if you move your arm, it registers a step. So if you stand there and brush your teeth without taking a step it will register something like 200 steps. As such, the steps need to be taken with a pinch of salt - as such, is there a need to spend £80 on a flex? No, you may as well just get a decent pedometer such as the omeron.
I find that it doesn't record the sleep accurately either. It has stated times I've been asleep, when I've actually been awake so its not reliable.
You don't get a warning on the thing itself when the battery is about to run out, when it does, it just switches off. Life lasts about 5 days of normal use. Also, its all done via synching to an app, so there's no way apart from 5 little lights that light up, to know where abouts you are in terms of the target you've set.
The food diarys on fitbit aren't great, but you can link to MFP and log the food that way and it then offsets the calories used from your exercise against the cals etc on the MFP app. So the app itself is great.
Someone I know has the fitbit one, which links to the above, but it clips onto a waistband or bra, and we've found this to be MUCH more accurate than the flex that goes around your wrist.
As such, I've done comparisons on my phone with an app and I have an armband which it sits in, and if you've got a smartphone you can get some great apps to go along with it and its just as good.
Would I spend or recommend a fitbit flex ? No. Motivationally wise, these things are great, but quality wise, reliability wise and the fact the flex gives such inaccurate results, I'd suggest looking into a fitbit one or even a decent pedometer such as an Omeron.