I use a Garmin FR220 and the HR keeps you honest. It's great to track your rides and interesting to look at the data. Some days I'm off pace, but HR is higher than the last time I did the same circuit. So I take into consideration weather conditions and the condition of the trail. Amazing how much a damp trail will suck you in and scrub lots of speed off. But if the HR was north of 130bpm, it's all good.