This is one piece of equipment that I recommend all my clients get. It is imperative that you know your heart rate when you are training to ensure that you are working at a sufficient enough intensity to cause a change in your body. The only way to know this for sure is to wear a heart rate monitor such as this.
All of my programs contain heart rate guideline for the cardio training sections, and trying to take your pulse manually is nearly impossible to do while working out (you will end up with major errors). Therefore all my private clients are instructed to get one of these.
I have seen many different models and prices of monitors, but my favorite continues to be the Polar F4. The price is great, it is easy to use, and it even tracks how many calories you burn during a workout so you can make sure that you are training just as hard each session.
The best price I've seen to date has been Amazon, and you can click the link below to take you right to it.