How accurate are those calorie indicator or measurement on Treadmills, Elipticals and Stair masters?

sandy asked: I would love to know…..I usually burn around 500 calories when I work out. How does it measure how much calories you burn?

Are there any special low cost treadmills that are only meant for walking briskly(and not running)?

Lahm212 asked: Can I use a regular treadmill just for walking?