I've noticed it's accurate on my 20-30 minute runs where I'm getting a HR of about 160 bpm, give or take.
However, where I am left scratching my head, is when the heart rate monitor is in resting mode where it records my bpm every 10 minutes. Between every 10-20 data points in that resting mode, my resting heart rate jumps from say 62bpm to something in the high 90s or low 100s for only one data point, then it goes back down to the correct number. I know this from the data contained in the Health App on the iPhone.
Moreover, I have swiped up to use the HR Glance screen to see something like your heart was 92bpm 2 minutes ago and when its done with an on demand reading it corrects itself to something in the low 60s, where it should be. Considering calling AppleCare about it or maybe just waiting for OS 1.1.