I got this DAC/amp a couple of days back.
I have the ODAC and O2 headphone amp, and this one is as good. In other words, it's absolutely clean and drives my 250-Ohm DT880 effortlessly. It gets warm as others have reported, and its volume control works very well. I need the high gain setting for the DT880, but all normal earphones which work with mobile phones will work happily with the low-gain setting, I'm sure. I specifically bought this to drive my large cans. I am driving it from my Linux laptop, and it works without any problem. As any Linux users probably know, no driver installation etc needed.
The physical build quality and the volume control feel is very good. I think it'll last a long time mechanically.
I am not sure this device is suitable as an outboard DAC for a mobile device, unless you just want to listen to a song or two. This DAC drains current fast, therefore your cellphone will drain fast. I think it's a great addition to a laptop rig.
I am an engineer and music lover, not an audiophile, so I may not hear differences between two pieces of equipment above a certain quality level. I feel that this device will probably match a Benchmark DAC1 or DAC2 headphone output, in case you want a guess.