Best-designed "tap" heart rate monitor out there. The tap method (as opposed to the iPhone camera methods) is good if you're listening with a stethoscope, especially with pets and kids.
My only gripe with the app is how the average is calculated: it takes into account the time leading up to the current "beat", so the average is always falling. This is ok if you can keep your eye on the app while listening to a pulse, but if you need to look away from the phone while taking a pulse (moving kids/pets aren't always easy!), it will be tough to get an accurate reading.
I suggest you add an option to take an average by including the previous beat duration in the average, which would eliminate the average constantly dropping between beats. That's how other heart rate apps are, and it'd be cool if your app could do both methods!
I'd change my review to 5-star if that happened, for sure!