The problem with GPS and heart rate and all the data that the little gizmo's give you is that they are short term metrics, that are generally only applicable to that day and point in time. Yet everyone wants to use them to change the way they train tomorrow, and it doesn't work like that.
Take your heart rate. Today you go out and you run a 6 mile out and back course, and it's windy and cold, and you end up with an average heart rate of 180. Four days after that, you run the exact same course, and it's windless with moderate temperatures, and your average heart rate is 165. Did you suddenly gain fitness in those four days?
Data is data. It is best collected and averaged out over long periods of time (months to years), and instead of comparing one week to the next, you look for trends in whatever metrics you are tracking. So if you are looking at heart rate, you want to see a general trend lower over the same types of courses/conditions as you get fitter. Or, if your heart rate is staying the same, hopefully you are running it in a faster time.
Conversely, if you track it and over the last 3-4 weeks you are noticing a rise in the heart rate, then you can look at factors that influence it. Have you been stressed, eating different, not sleeping, overtraining, etc. and correct it.
The reason that I stated the only metric that matters is race times, is that you can train according to the numbers all you want, but if your race times are not improving, then either you have maxed out your abilities, or there is something else that you need to consider.
FWIW, I track mileage and training times, and I track power with a powermeter on the bike, but I've never tracked heart rate at all. If they ever come out with a commercially viable powermeter for shoes (There are some attempts out there, but I'm not impressed yet) I'll be the first one in line, but HR (to me) just isn't a great metric, there's too much that influences it. However, other people swear by HR based training, so your mileage may vary on that one.