When Jillian Michaels had a radio show she talked often about how they used polar heart rate monitors with the Biggest Loser contestants for the first few seasons. She said she always found them to be very accurate and could estimate their weight loss for the week within about a pound (which, considering what they burn in a week, I'd say is accurate). The only inaccuracy would be that it doesn't account for your percentage of fat vs. lean muscle mass, which would vary between individuals.
I love my Polar monitor and have found it to be pretty accurate. As long as the band's sitting flat on your ribcage and you've plugged in all your stats you get a good reading. If it's slipping and sliding during exercise then your burn will probably be low. If nothing else, it tells me if I'm exercising in the right "zone" of intensity and I know if I'm getting the kind of workout I want.
I really couldn't workout without mine now