As far as I can tell it doesn't make apps private, just data, which is fine by me...keeps the girlfriend from finding the mistresses photos by "accident".
The one thing to take note of though is that it appears to move any data to a secure folder on the phones internal memory only. If your data is on your SD card and you make it private it is moved to secure internal memory.
That folder only appears (even when connected to a computer) when private mode is on.
It's not perfect but if security is a concern it seems to work very well. Me likey.