Empirical evidence on an average does not mean that an individual anecdote is false. I can echo cbanek's finding - I've been a competitive runner for 15 years,

Empirical evidence on an average does not mean that an individual anecdote is false. I can echo cbanek's finding - I've been a competitive runner for 15 years, and there's a small inflection point in my results about 5 years ago when I got a GPS watch and heart-rate monitor. I was training and racing to the best of my abilities with the tools I had before, and when I got more abilities with the watch, I did better. I suspect the difference is that some use the device as a crutch rather than a tool.