Tried, tested, they failed.
Seconic 308's go for less than £80 and will give you the correct exposure that you want. Although the apps are not too bad for natural light, they still result in 1/3rd of a stop out so that could completely ruin the shot all for the sake of relying on the camera of a phone, plus as Phil has already mentioned, I've yet to see an app that does flash metering (unless it records until you fire a test shot and records the spike in light) but why go to all that bother?
There's currently 2 professional versions for the iPhone but for the price of those you may aswell buy a genuine unit!