I just bought this recently and so far  I am very impressed by the quality of this unit. It's a very capable unit capable of many things other than just playing music. The DAC mode is good for anybody wanting to upgrade their sound interface on a laptop. There is 4 ways of output,USB device into PC/Music system, headphone (Low and High Gain) and Line Out and Digital Coax options. It is also capable of decoding DSD in file format using DSF downloads at 2.8Mhz/5.6MHz (64/128 fs) or SACD ISO files through software or on the device itself. Although I haven't been able to try the latter as yet.
 
The UI is a little basic to say the least however it does what it says on the tin. It shouldn't take long to learn however this is no iOS or Android (technically it is) interface in appearance, it seems more like a Symbian environment without the Java. When you just want to play music you shouldn't need too many options. There's not much in the way control although it does feature an equaliser, stereo balance and favourites tool for keeping your best tunes in the one place.
 
You can research the parts such as the DAC and Amplifier components elsewhere. A quick summary would be it will drive a lot of headphones such as sensitive IEM to Studio reference headphones under the $500 bracket with no issues. The stated range is up to 150 Ohm headphone resistance which will cover a lot of options. I tried my Shure SE425 IEM with them and the sufficient gain really makes them shine. Also gone is the electronics hiss I was getting from my mobile phone. Then I tried my AKG K702 on High Gain which use 62 Ohm and 200mW drivers and is harder to drive due the AKG driver design although it's clean and impressive for a small player.
 
Music auditioned:
 
Pink Floyd - The Endless River (24/96 FLAC)
Chase 'n' Status - No More Idols (160K MP3)
The Orb - Blue Room (40 Minute mix - 128K MP3)
No Doubt (256K M4A)
Emeli Sandé (320K MP3)
 
and some other FLAC 44.1K/16 bit encodes direct from my CD collection.
 
To create some DSD files I used Cakewalk Sonar Platinum which exports 1-bit 2.8Mhz/5.6Mhz formats by using the SSE3 found on CPU processors. This information was given to me by the CTO Noel Borthwick although the actual DSP codec was created by someone at Tascam.
 
If you do prefer your other music interface then you can use it as a DAC and bind them together with a USB OTG cable which will turn your mobile device into a portable hi-fi device, if you don't mind the chunkiness. There is a few options of connection and is compatible with Windows 7 onwards and Mac OS X. I'm not sure about Linux however there is always a workaround there.
 
 
So would I recommend it?
 
Yes, if you don't mind the fact it has no internal memory and relies on external SD. Officially you are restricted to 128GB although maybe this limit will be lifted when more Fiio products become available. The next model up X5/X5 ii offer two slots meaning you double the potential and a slightly improved amplifier stage. I can't fault it for the majority of things I have currently thrown at it. The TF/SD card slot threw me at first. The picture shows a straight insert of the memory card however the actual insertion requires snapping the card in at an angle.
 
 
To summarise, this is a great piece of kit and great introduction to the world of high-end portable players.

Background information:
 
I am a hobbyist, part-time geek, audio enthusiast who likes high fidelity gear (or just whatever I can afford on the day). You'll find me messing around in Sonar Platinum, EWQL Virtual instruments and other wonderful music production programs. 
 
For reference music: I regard Pink Floyd, Fleetwood Mac, Mike Oldfield, Brian Eno and other progressive rock as a benchmark point. Also some classical recordings are a good way to evaluate the sound quality of some headphone products. Neutraility suits me best as I can always add and subtract sounds with an equaliser. CD Audio, DVD Audio and FLAC are my go to formats for music or at least MP3 320K.
 
 
Equipment used:
 
Self-built PC using EMU 1212M soundcard
Samson C-Que 8 4-port stereo headphone amplifier
AKG K702 (None Anniversary edition)
 
 
About the headphones
 
These use large semi-open head-cups which will mean they are not ideal to use around recording microphones or public transport. The claimed frequency range of each driver claims to be 10Hz-38.9Khz. Which means they nearly fit the Nyquist ideal of double the frequency for things to be heard properly. The K812 Pro have a reading of up to 44.1Khz due to their larger drivers and 1.5 times factor of the Tesla magnets.
 
The impedance of these K702 headphones at 1Khz are 62 Ohms, a sensitivity of 105dB SPL/V with a maximum power of 200mW input. Despite all of these relatively low figures it does mean they do need a lot of amplification to get the best out of them.  Once these are properly amplified they will come to life, so a run of the mill Digital Audio Player or generic smartphone headphone amplifier won't cut it really and they will sound thin.
 
So who are these headphones for, audiophiles, studio engineers or your older parents with failing hearing?
 
Audiophiles will argue they lack low-end and accuracy in the treble whereas a studio engineer will argue they offer a neutral reference for between takes however not for critical mix decisions. I don't think any headphone can offer that as a solution really.
 
Playing well recorded masters will reveal a great amount of detail and soon you'll find yourself hearing things you never noticed before, like amplifier noises or coughs. The soundstage is pretty detailed and the louder you turn them up more harmonic content becomes present.  Adding a touch of low-end with an equaliser of say around 3dB should be enough make them good enough for most music when needed. This is a weak point addressed in the K812 and K712 Pro which should give a more rounded sound rather than a bloated overpowering emphasis on the music.
 
Evanescence: My Immortal really comes to life (ba-doom-tish!), revealing much more of the stings and piano resonance not that obvious when listening on cheap headphones. 
 
Pink Floyd: The Division Bell (24/96Khz FLAC) playing through 24/96 ASIO driver sounds pretty flawless where you can almost hear the strings of the guitars being strummed or maybe I'm just hearing it with far more clarity. Reverbs and room impulses seem to be more apparent.
 
 
The only dislikes I do have are the headband can be uncomfortable after an hour of so, how they reveal of the mistakes of your favourite recordings (not really a dislike as such) and the fact I paid more for mine and soon after AKG reduced the cost. 
 
Overall I would recommend these to people looking for accuracy for a good price.

I had these a while back and found them to be pretty good alas they did have a couple things that spoiled them. 
 
Comfort: Finding the best earbuds took a bit of time. Although once you did, they were comfortable enough.
 
Sound Quality: These have a tight-bass extension, providing enough energy for most music. Some hip-hop fans might not like their politeness when it comes to phat bass however. The treble especially in dance/EDM often sounded a bit bright unless you used an equaliser to take the edge of the sound at 10Khz and beyond.
 
Design: Awkward Fit. Otherwise OK.
 
Tech specs: Sensitivity of 115dB @ 1Khz (32 Ohm Impedance) 
                    In-Line volume control
                    Carrying Case

Ok, I've had these for a while now and decided to leave some feedback on these. First thing, the sound from these IEM's (earphones) are nothing short of excellent with a few minor dislikes.
 
At times, some music demands a bit more low end although to be fair that is what an equaliser can be used for. Like any good quality headphones, they will reveal any bad traits in your music source; so often, what sounds exceptionable on more tuned earphones will sound flat out trash on these. The fit can be a bit fiddly and it takes a bit of time to locate the best buds for your own ears although the supplied kit is plentiful. 
 
After using these for a little while, you may start to notice small nuances in familiar music you never with cheaper or alternative earphones. It's a dual-driver design with one mid/treble and bass driver each ear. One thing that occurs to me, the lacking bass power can lead vibrant mids although with a declining treble starting under 17Khz could explain the highs not being as present. Once the drivers have been run for a while, you notice your ears will warm into the detail produced by these.
 
By comparison with my older Sennheiser CX880 which also did a commendable job, they also had problems with "too much treble" in some modern music or badly mastered digital. So often this was corrected by using an equalizer on the Creative Zen player. 
 
Listening to soundtrack music on the SE425 shows how detailed and the soundstage of the IEM by the way instruments are panned & placed virtually "in your ears".  In direct comparison to AKG K702 they put up a good fight. Which is ironic because the retail more now and have a lesser output.
 
For the moment, they will do the job although I do hope to get an extra higher grade pair at some stage.

These have nice dynamics and are flatter than some headphones. Despite that the low-end still has enough gusto to bring tracks to life. Another benefit is the detachable cable which uses a 5N OFC (99.999%)  make-up meaning the likelihood of internal corrosion is reduced further. As this cable is long and the headphones uses open back means they may not be ideal for travel situations like the bus or train. 
 
To be honest, whilst these won't replace a good pair of Sennheiser, Grado, AKG, Shure or Audio Technica headphones they do a fairly decent job in my opinion. 
 
They use a 32 ohm impedance although I don't know how much power or the actual sensitivity although I suspect it to be <100dB SPL due to the gain required on my headphone amplifier to gain an average listening level.