Plugged into my PC fixed with integrated sound card, and a low-end headphones:
The difference is incredible, whether with FLAC files, or even with online games like Battlefield 4, the difference is really meant.
A true happiness.
On my Nexus 5, CM11, with medium-range headphones (Marshall), I am a bit more reserved.
AUX course we lose the interest of the DAC.
But even plugged in DAC with USB OTG cable, listening to FLAC with PowerAmp, the sound is good, but the gain is less noticeable than on my PC. Good but not great.
With more space (Nexus + DAC + cables), now I prefer without.
But overall still a good product good quality.