I've had my Fitbit for 6 months, no durability or battery issues so far (it recharges via a USB connection).
I like that I can sync it wirelessly to my iPhone and see a nice summary of my activity. I don't use it to track
calories. Mine has a heartrate tracker, my doctor compared it to his heartrate monitor in his office and it was
reasonably accurate. I have a heart arrhythmia that comes and goes so the daily chart that is produced has
turned out to be quite useful.
I find that it does motivate me to move more.
It can be used to track your sleep as well, but I find that having the plastic wristband on 24/7 is too irritating
on my skin.