I've worn a Fitbit Zip (almost) every day for two years now. My thoughts:
1) If you don't wear it, it won't do any good. I don't like things on my wrist, so I won't wear them.
2) I went with the Zip partially because of battery life. If it's plugged into a charger, it's not on me.
3) Fitbit's app is easy and intuitive. Don't have any experience with other brands.
4) The only one you are really competing with is yourself. The Fitbit helped me realize that I was at different times more and less physically active then I thought I was.
5) Some of these devices will flood you with data. Heart Rates, Sleep (Really? When do you recharge it?), GPS, Activity, etc. I think starting with a basic device and seeing how it goes is the way to go. You aren't out a terrible amount of money and can upgrade later if it's what you want.
5) Distance measurements on non-GPS devices is marginal at best
6) A trip though the washer and dryer is good for about 5200 steps. Don't ask me how I know this...
7) Finally, a basic pedometer and an Excel spreadsheet isn't much different from a Fitbit Zip. It's not the device that is going to make her loose the weight - it's what she does with it. A good running partner did more for me than any gadget.