The SRM Training System was the first power meter for road cyclists on the market. SRM uses a special crank system which measures the workload you apply. Thus, it is a direct measurement of the workload. That makes a lot more sense than the indirect methods used by e.g. Ibike Pro.
Several years of experience
SRM has had several years to optimize their products with valuable experience from top riders and scientists who have used SRM as power meter. As I have mentioned earlier, one of the best ways to learn more about power meters (or heart rate monitors for instance) is to spend several hours on the roads with the power meter mounted. The more data you can accumulate, the better picture you will get of your performance and what a power meter can do for you. Reading about training principles and basic exercise physiology will naturally help on your learning curve, but don’t underestimate the value of practical experience.
I think that SRM has a huge advantage against the new generation of power meters like PowerTap. These cranks are really good, comparable with stiffness from the best cranks on the market (though they are slightly heavier). A crank is a part of the bike so there is no problem when you e.g. want to put on your new carbon wheels for a special race. The SRM is still there and works properly.
SRM is the gold stand for power meters with very accurate measurement of workload. It’s a high quality product that will not disappoint you.