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.