I have one and it's brilliant. However it's not so good for exercises which aren't walking based (cycling, weight lifting, cross trainer) so you would need a heart rate monitor to track your calories burned. I've lost 10lb with it so far and been using it religiously for about 2 months. I've had it a lot longer but didn't really stick to it until now.
It syncs with your phone or tablet, gives really detailed info but it does take some time for it to "get to know you". Battery lasts about 5 days before needing a recharge.