I used it to back up my PSP Go PSN games....it's really simple and easy to use, as dictated by the readme -
Locate EBOOT.PBP of the game to decrypt and change its name to NP.PBP
Copy EBOOT.PBP and npeg.prx from this release in that folder.
Run NpegDecryptor. The game will be decrypted to the ISO folder with some name as "NPEG-XXXXX.iso" "NPUG-XXXXXX.iso"...
After decryption, you can play the iso game normally as you would do with other iso, try various umd modes if one don't work.