It's not a HR monitor, but I use a device called a bodybugg (
link) - you strap it to your tricep and it measures your body temp, movement, and galvanic response (basically sweat) and calculates your real-time calorie burn throughout the day.
I've been using it off and on for over a year, and while I can't say it's specifically helped me lose weight, it's NOT because of the device (i.e. it's my own commitment, etc). It's a great tool, though - it's shown me how much I really do burn when I'm active (up to 3500-3800 a day) and how little I burn when I'm lazy (some days as low as 2000 or so).
It's not cheap - $400 for the armband and the first 3 months' service, and $15/mo or $99/year for continued service, but it's been worth it to keep myself realistic - I know that eating 12-1500 calories a day doesn't work for me when I'm burning 3000+, but my "diet brain" still argues with me when I eat up to 2000 or more. This information helps me keep myself sane - I can look at the computer and SEE that I've burned plenty, that I've still got a 700-1000 calorie deficit for the day, and that it pays off when I weigh in.
If you want to know more precisely than the usual estimates (even readouts on treadmills, etc are still estimates), this is one heck of a tool to know exactly what your body's doing each day. Since weight loss is calories-out-vs-calories-in, it's pretty easy to keep on task when you KNOW both numbers!
Caloires
