It would be cool if someone with a real heart rate meter could run some tests to see how accurate this is. For what it's worth, I couldn't get it to work. The values fluctuated between 50-150 and refused to settle on any value.
On the android app, the heart pulses to your heartbeat. I checked my pulse simultaneously and it seems pretty accurate.
Seems to work perfect for me; it reported 68, and I counted 66 manually (by counting the number of beats myself in 10 seconds and multiplying by 6). For reference, I have an iPhone 4 and it utilized the flash.
Only counting 10 seconds worth reduces your precision pretty substantially. I generally count for a minimum of 15 seconds, with 30 being typical.
In any case, the software also shows a visual indicator of when it detects that your heartbeat, and it graphs your heartbeat over time. It's obvious from feeling your pulse while using it that it's correct, at least in my case with my phone.
2/3 of the people who responded to the poll on the site say it's accurate but I wonder what equipment they have on hand to let them know that what they believe to be the rate actually is.
I think if people are resting and see something between 50 and 80 b/m and then see 120-150 when exerting themselves, the device is probably accurate. It doesn't have to be perfect.