The only catch is that the machines are constantly telling me to slow down to reduce my heart rate.

If you are working at a cardio zone, can you still burn fat, even though at the cardio rate you are at a higher heart rate?

This is my understanding of how things work: When you are working in the cardio zone for you - after about half an hour - you will have used up the sugar that is immediately available for fuel in your blood and then you will start to burn fat

I think you should pay attention to the machine provided you think it is reasonably accurate. As you become more fit, the intensity of your workout will increase because your heartbeat will slow down. I absolutely use my pulse rate as the measure for going at things too hard or too soft. It is the only measure that is unique to me.

I also use my pulse rate as a measure. According to what I've read, if you get above your cardio zone, you will be burning muscle instead of fat. And as Devorah says "As you become more fit, the intensity of your workout will increase because your heartbeat will slow down."

I purchased one of the heart rate monitor/wrist watchs. While wearing this on the elliptical or treadmill at the gym-it reads my HR constantly (on the machine and on the watch) without my having to keep putting my hands on the HR monitor on the machine. The wrist unit beeps if I get above my zone. This is very helpful and more comfortable than having to keep putting my hands on the bars.