I use it everyday for ummm.....special tihngs. No seriously, My wife is in South Korea for a year and we like to send some pictures back and forth that i would rather people not see so i put them in albums and mark the albums as private.
It works great, no need for an app to hide private files. I also have movies that are private. And it was pretty easy to transfer the stuff that i wanted on my phone using the app on my mac when i got the phone two weeks ago, it allowed me to copy everything right into private and make albums, and then i am adding new stuff to it all the time. Whenever i want to add to it or just browse, i turn on private mode, it requires my password or a finger scan, wait about 10 sec and everything populates in my gallery. I have made the private mode button on my settings in my notification bar for quick access. That is absolutly one of the best features of the S5 in my eyes.