I've used it in three of my four marathons. The key though seems to be predicting a realistic target - I finished my second fastest marathon comfortably using Marco with a five minute negative split. I then tried to go for a faster time with the same method and I just couldn't speed up enough in the second half so finished with a small positive split. It was still a PB but not quite what I was hoping for. This was at Chester where there is a bit of a hill in the second half. I am a big fan though, it gives you the confidence to hold back first half with a real plan and seems to make the experience much more pleasant!