I suspected that assuming you are used to checking your BPM regularly apps which rely on the flashlight and camera weren't more precise than a manual check. Event if they rely on a technique used by medical apparatus.
But then I did some testing.
Comparing with my own readings the results seem to be consistent with a ~5 BPM margin based on the approximation due to the multiplication. I actually am less precise than these apps.
2 different iPhones models with flash
The results of some testing using 2 different iPhone models, the iPhone 4 and the iPhone 4S shows that on 6 readings (3 per device), I had a difference of 2 to 0 bpm with less than ideal conditions.
vs a Gamin Forerunner
Testing of one of the apps agains a GPS sports watch with BPM Showed very close results on a dozen readings. A 3 BPM difference was be the least precise it would get in when testing in good conditions, most of the time they are off by 1 or 2.
Convenient and precise
If you don't press hard on the camera (to avoid blocking blood flow), remain calm, not moving and there are no strong variations in lighting around you you should get a decent reading without having to concentrate on feeling the blood flow and counting the heart beats.
Apart from the convenience, I believe the real advantage of these apps is the tracking and history of your bpm.
If you want to use these apps before and after exercising they will be enough. Based on my tests, the accuracy seems very good (better than I believed) if you make sure to place your finger properly, covering the camera without pressing hard as not to stop the blood flow (again this is important).
I think I will trust the iPhone from now on ;)
As a side note. If you want a really good way of keeping track of your bpm and blood pressure this product works great on the iPhone/iPad.