The app does what it says, but not in live time. You take your photo (or upload from your Camera Roll), the app then analyzes it, then applies the blur. It takes about 15 seconds to do the process on each photo (iPhone 5s).
Ive used other apps that produce the same result, and this does a good job on its own that you can tweak afterwards. However, I cant justify the price when apps like AfterFocus are faster with more features.
Trevor the Canadian about FabFocus - Portrait Mode Blur, v1.0.1