I recently got myself a Fitbit Flex. I love it, and I do think it has helped to motivate me to move a bit more.
On the 'Dashboard', I log my food intake, and keep track of carbs that way. You can also 'add a note' to meal & snack slots, so I use that to record blood sugar tests. That way, I can see instantly which foods are causing a spike. Really pleased I bought it.
The clip that holds it closed is made from surgical steel and contains Nickel, which I am allergic to, but so far it has not caused me any trouble. I think the knack it to wear it slightly loose, so that the clip is not in contact with the skin.
It's quite amusing when I go out for a walk in the evening to top up my steps if I haven't reached 10,000, and I see several other people marching purposefully round the block and checking their wrist occasionally. (I call it the FitBit Club, and I have got on chatting terms with one or two of them!)