Like another poster, I found that the HRM was counter-productive in that it had me running slower than I thought I should. I may be wrong about my average marathon HR. And I know that I’ve averaged higher, when I wasn’t in peak condition.
I found the HRM to be effective in providing instant feedback, instead of waiting for mile splits I could maintain the proper effort late in the race when I was inclined to slow down. Actually, I was a bit surprized by my HR dropping as I assumed that it would rise as the effort increased. Just the opposite. It would only take a 10 second PM dropoff for that to happen and the HRM was helpful in making sure I pushed the pace. “Cranking it up” late in the race isn’t an ez thing to do. I would be very happy to just maintain my goal pace.
Perhaps I wasn’t using my HRM correctly, but inspite of that I still found it a useful tool.