Heart rate monitors can help you get fit. You can measure your cardio fitness level with a HRM. They can also help you burn a minimum amount of calories. They can numerically tell you how hard are you working out, independently of how hard you think the workout is. So they are useful. Just go for a name brand, and make sure that you get one with a chest strap to be able to continuously monitor your heart rate.
Well, all the above information assumes that you are into fast paced cardio workouts. Some people only walk, and probably they cannot make much use of a HRM. Other people who only strength train with very little cardio (usually just to warm up and/or cool down) probably won't need a HRM either.