I bought and started using a bodybugg almost 3 weeks ago!! I LOVE it!! It has really helped me stay accountable and on track. I know exactly how many calories I burn on any given day and then I log my calories into the bodybugg software and know what my deficit is every day.
My biggest problem has always been that I am very black and white about the diet thing. 1 cookie and I've "blown" it and might as well eat the whole box and have fast food for dinner. Now I see that OK, my deficit isn't as big for the day, but I still HAVE one or maybe I've broken even, but it's not the end of the world or the end of me being on plan.
And I am learning that my body doesn't care how I burn the calories, as long as I burn them. Last night I cleaned the house to get my extra calorie burn for the day instead of going to the gym. It all counts, its just that with the bodybugg you really know how much.
There are 3 kinds, sensewear, bodybugg and gowear fit. They range in price and have different features. Do your research before you buy. The thread they mentioned is the current one and this closed one
http://www.3fatchicks.com/forum/showthread.php?t=146354 has a lot of really good information.