I have a heart rate monitor and I use it every time I workout! There is something motivating about seeing the number of calories I've burned. It keeps track of my workouts (date, time, length, calories burned, % fat burned, etc.) and it is also a great way to challenge myself. I know my AT (where the body switches from buring fat calories to sugar calories) and try to stay right under my AT for the majority of my workout (to burn the most fat) but then spike above my AT to increase cardio endurance.
I would definitely recommend a heart rate monitor.