I love using a Heart Rate Monitor. I do mostly interval sprints for my cardio, the monitor lets you know when rest is over and its time to go again. Its good for keeping your brain from telling you its working to hard or to light.
I've been winging it but I really should bust out my heart rate monitor. I use it if I go road cycling mainly. Good investment for cardio workout, but it's pretty easy to tell if your pushing yourself. When using the heartrate monitor I could tell approximately what my heartrate was without looking at it. It's just a number to push yourself towards. I think it fits good with a weight lifters mindset .