By itself, Private Mode is an excellent tool with which to hide selected files. However, it won't prevent access to your phone logs, texting or chat sessions, email messages, and the like. As one of its dialog boxes reminds you, Private Mode doesn't have to be the only security method that you employ to protect the sensitive data on your phone. It can be used in combination with a secure lock screen method and/or encrypting the phone, if you like. Pick a combination of security options that affords you the best protection, while requiring only as many unlock requests as you can remember and tolerate.
Steve Schwartz is the author of 70 books on technology, computer software, and games—including detailed guides for the Samsung Galaxy S iii, S 4, and S5. Private Mode is also discussed in his most recent Que title: My Samsung Galaxy S5.