I've read lots of advice on the forum stating that you shouldn't race / run a marathon with a HRM - well I ran berlin marathon on Sunday with a HRM, having trained with it for the last year, and it was brilliant. I did the first 2 miles at 60-65%, then stayed at 75% until the last 5 miles when I stepped up to 80% for the last 5 miles. I've had a few injuries and a cold for the last 3 weeks so it really helped with the pcing. I mantained near-perfect pace the whole way deviating from 10min 35" miles by only a few seconds according to the 5k split times on the web-site. I finished really strong and according to my biased husband anyway looked much stronger going over the finish than most others I was finishing with. I love my HRM anyway, but now I'll never hear another word said against it!! I said before I'd never do another marathon, but I'll definitely do another, I know I can do under 4 hours next time!