I can confirm that the Fiio E18 Kunlun works as a DAC flawlessly with my AT&T Samsung Galaxy S5 Android phone running factory stock firmware (4.4.2). It works as a DAC with music apps I've tried including Spotify (saw lots of people asking about spotify). [Play/Pause], [Previous Track], and [Next Track] also work in DAC mode with Spotify and other music apps I've tried.
On PC:
When I have the E18 connected to my PC by USB and I'm using it as an external DAC/Sound card (Windows detected driver as "USB Audio Device" "SPDIF Interface") I noticed an issue with distortion in producing notes (especially highs) in music, a substantial buzz/hiss at all times, and clipping in and out at times. Was very evident when running the Windows sound test through the Sound Playback Devices Control Panel. (plays a loud ding through each audio channel.) I noticed this on both my headphones and when using the line out into my speakers from the E18 DAC.
I found that the distortion/clipping/noise only occurred when the Fiio E18 was set to "PC IN" on the bottom 3-way slider switch. Setting the switch to "PHONE IN" resolved the issue completely. When set to "PC IN" the Fiio E18 will also charge over USB. It seems like there is a lot of noise produced by charging. I'm curious if the noise is actually coming from my PC and carried over the cable, or if the noise is produced simply by the Fiio being in charge mode. I don't have an extra cable to test this with right now, but I may try later.
So basically don't charge and listen at the same time, use PHONE IN instead of PC IN. Probably could of explained all that without being so verbose, but I like being specific to rule out questions...