Don't get me wrong, I'm not selling it as a neccassity, but coming from training with HR and going to PM, I prefer the PM For instance with my tri bike, my HR actually goes up when I come up out of aero, while my power goes down. Weird, and I don't know why.. Also, I find it better for pacing myself during a triathlons. during my last race (70.3 tri) my HR was jacked for the first :45 of the bike. I stuck to my plan, and the target power I was supposed to keep, but my HR was still bouncing between 155-160. After about :45 I settled in, and my HR came down to the 130's where it was supposed to be. If using HR, I would've been bagging it for almost an hour. I don't know if it was the fact that I was really hyped up about this race, or if I overdid it in the swim that caused that, but it was weird. Also, my HR does funny crap in the cold temps.