I personally don't think a heart rate monitor is critical to a good workout. I've been working out for well over a year, have made excellent progress, and have never invested in one. It is a nice to have, but they are expensive. I'm not saying that you shouldn't get one if you want one; your heart rate is helpful info to have so if you do want to spend the money on one, by all means, treat yourself. But if you don't want to spend the money on it, I don't think it's critical to getting a good workout. I use the "perceived exertion scale" and it's worked well for me. You can learn about it at http://exercise.about.com/cs/fitness...ceivedexer.htm
As your cardiovascular health improves, you are going to have to increase your workouts to get the same level of benefit. You can either work out longer or work out harder in the same amount of time. My vote (and my personal trainer's recommendation) is to work out harder in the same amount of time. Really, how much time to you want to spend working out anyway? If you can accomplish the same benefit in less time by working out at a high intensity, why not do that? That's an advantage of interval training--you are able to work out at a really high intensity, so you don't need to work out as long.
My Before and After Pics
"Decide what you want; decide what you're willing to exchange for it; establish your priorities, and go to work." --H.L. Hunt
"Life has hills. Set the treadmill at an incline."