Its very accurate since you are wondering, you will see many comments in the iTunes app store about how it was exactly the same as Polar Heart Rate Monitor.

The iPhone amazingly enough does not really have a hard time at all, if you place it correctly on the neck or chest the heart beats actually max the micrphone out.
I absolutely concede that if you are in a very noisy environment and you have a weak pulse it could have troubles.
But it does a lot of very smart things to filter out noise, erratic beats or missed beats and doesnt provide a result until it is statistically confident that the samples has got are consistent (eg Standard Deviation falls is very small between each sample).

I do invite you to try it, this app was 5 in Italy for a long time and has been in the top 100 in the USA so a lot of other people have found it really good.

Cheers, John.