I am a big believer in using a heart rate monitor when working out. It really can make a difference in your training and is a necessary tool if you want to achieve your fitness goals. I bought my first one in 1989 and have been using one ever since. At that time there was only one brand that was readily avaivable and to this day they are the most well known and respected in the industry. That would be Polar. Polar invented the first wireless accurate EKG heart rate monitor back in 1977 for the Finnish National Cross Country ski team. The one I bought in 1989 was made in Finland and the reciever was a huge bulky square watch. Now they come in an assortment of styles and have all sorts of optional functions, bell and whistles. I use the S710 model which is bike specific. Besides monitoring my HR, it can record cadence, speed, log miles, record time and is all downloadable to my computer where I can view the data via charts, grafts, and numbers. How fun!
Maybe not every one needs to have all those features but certainly if you want results (whether it's weight loss, improved cardiovascular performance, or just be accountable for maintaining a healthy heart),I suggest you invest in one.