[T-Mobile Samsung Galaxy S 4] AlaskaLinuxUser:
Glad to hear that the new rom is working great on your end!

So, I made a backup of my phone, and took the fateful plunge to try out the encryption. It took forever to encrypt my phone. After an hour, I went to bed, and it finished sometime in the night. So when it says that it will take greater than 1 hour, it is right.

Second, it will not allow you to encrypt your phone unless you have a fully charged battery AND you are plugged into a power source.

With this in mind, I proceeded.

First attempt was a fail. I realized later that you must set a pin lock or password PRIOR to encrypting your device, or when it reboots the final time, it cannot be unlocked and sits at the boot animation indefinitely.

After seeing it in action, it seems to be somewhat problematic. Some apps just don't work anymore, and I do not know if it because they now lack permission to read something, or if the encryption is faulty, or if they are not on the key-ring to read the encrypted files. What I wish I had tried was installing from the Play Store. I have already flashed my phone back to normal and it isn't quite worth the effort to try just that.

To revert back was less than a civil affair between myself and my phone. I am not an angry person, but my phone just about took up flight school a few times. To get back, using wipe was not enough. Using advanced wipe was not enough. Formatting to ext4 again was not enough. You have to format to something else to really wipe it. Something like ext3. Then format back to ext4. That worked for system, etc, but even then it still didn't work on the Data partition.

For some odd reason, using advanced wipe to format the Data partition was not enough. In the "wipe" menu, there is a special button for "format Data partition" after which, you must type yes, then press enter to complete the action. After doing all of the above, I could then successfully restore my phone.

In either event, I would recommend NOT using full phone encryption. This may cause problems for our phones in the future, as Marshmallow is mandating encryption if your phone hardware can support it. I am pretty sure that Nougat will mandate encryption completely.

However, if you are worried about phone files that are un-encrypted, there are several apps available to help with that.

I did try a few different ones, to help my brother, but I will share here in case anyone is interested:

LUKS manager - Requires kernel support through CONFIG_DM_CRYPT=y , and the AKLU kernels all have that flag set, so that is not a problem. However, the LUKS application does not work in Marshmallow. So LUKS manager did not work.

Encryption Manager lite - This one did work. However, the lite version only allows the encryption of 5 files. So I don't know if it is worth paying for, and the interface was not intuitive, but it did work to encrypt files and folders when I tried them. The files/folders that are encrypted are also hidden from view by other apps unless you "share" the encrypted files/folders with that app.

APG - This app worked the best. The interface is kind of limited, but you could encrypt any files (as many as you want) and use keys or passphrases to unlock them. You can also add "trusted apps" - I didn't try that, but it seems that they are allowed to read the encrypted files.

So, I hope that helps, and again, at this time I do not recommend full device encryption. If you do decide to use it, let me know if you have a successful setup. I have a sneaking suspicion that a clean install, then enabling encryption, then adding all of your stuff may be the way to make it work correctly.