HRM's are probably the single best way to help you out with training. because they are readily available and cheap (compared to a powertap wheel or SRM cranks, anway) it makes them a feasible part of your cycling budget. HRMs show how hard YOU are working, they dont give you some irrelevant # (like trying to compare your watts to Jan Ulrich... please) HRMs let you pinpoint how hard you work on a given workout - low HR on recovery days, high intervals on hill repeat days, red zone on race days, and stuff like that. moreover, they help you recover smart, too. taking your resting heart rate and using something like the 'polar fitness test' can give a really good way to track recovery and progress. and probably the most useful function for me is the software that came with my Polar s720. i can download my workouts and log info into the digital training diary. you can look at the HR zone breakdowns per week and look at total miles, time, elevation, etc. so youve heard my song and dance... hope it helped.
You are incredibly wrong regarding your analysis. HRMs do not show you how hard you are working -- HRMs only show how fast your heart is beating, which is affected by many factors including heat, hydration, medications, effort duration, emotional stress and rest.
A watt (actually a joule) is the only measure of how hard you are working, and the number is not irrelevant.
300 Watts is 300 Watts, no matter how hot, windy, or hilly it is, or what your heart rate is – though it may “feel” easier or harder, depending on various conditions. A measure of watts is the single most relevant data point for a cyclist.
Tracking wattage is the only way to determine inter and intra season improvement; heart rate measurements cannot provide such information, neither can speed. The importance is not comparing yourself to Jan Ullrich or anyone else. You use power data to compare yourself from week-to-week and from year-to-year.
Regarding software, Polar software is OK, but there is far better. Cycling Peaks Software has a superior interface and provides algorithmic methods to track workout intensity based upon kilojoules, duration and your current functional threshold power. You even get a truly accurate measurement of calories burned during a workout.
I agree that Power Meters are expensive; however, given you already own a Polar S720, the power option at $300 is not prohibitive and my experience with the Polar PM is pretty good -- when properly setup they compare very well to a Power Tap Pro. For $300 you can also shave a couple of grams off your bike weight, the choice is yours.
Keep in mind that a power meter alone will not make you more powerful; its only a tool to maximize your training efforts and physical potential. In addition to what I mention above, power meters can do the following:
1) Pinpoint strengths and weaknesses.
2) Test position and aerodynamics.
3) Perfectly pace efforts.