Pros: Tiny size, tiny price, easy compatibility, fabulous natural sound
Cons: ? yet to be identified
On receiving the DAC I set up a Raspberry Pi to work as my server (for mpd, samba, nfs, dictd, sshd, bittorrent, get_iplayer) but it turns out that the Pi's hardware is not as good as the spec suggests so there are latency problems with USB and networking, and problems with ground loop hum even when using USB DAC and it was all a bit frustrating and I never really got to relax and enjoy the DAC. Then I tried the Hifimediy ESS DAC with my EeePC. Holy ESS AICH AYE TEE!!! (good pun?). My Eee PC 1001P has pretty decent sound for a laptop but the Hifimediy took it to some other place. It sounded superb. Not just relatively good but actually good. OK, Raspberry Pi got relegated to dust collection duties and I repurposed an ancient (2007) Dual Core Celeron laptop as mpd server (it's silent and has a battery so will shut down gracefully in case of power outage) doing all the tasks mentioned above. I also bought a shielded 10m extension cable that can sit between my Momentums and the Hifmediy and lets me shuffle around my small apartment with headphones on. The Hifimediy sounds outrageously good. I set up the mpd server so it outputs bit perfect audio via the Hifimediy, or I can choose to run audio through a bs2b crossfeed before the DAC, or by pass the DAC and output flac/mp3/ogg/raw pcm streams to my LAN (or SSH tunnel). I choose bit perfect for natural sounding headphones like Momentums and Koss KSC75 and Meier type crossfeed for IEMs or anything which needs a bit of help to sound believable (and flac or pcm streams to Android according to device capability). The Hifimediy has enough voltage to power the 24 ohm impedance Momentums (I have the circumaural version) really nicely. It should have no trouble at all driving any IEM you might like to connect. If I feed the audio into my FiiO E7's line in I find the Hifimediy + FiiO amp sounds clearly better than using the FiiO as both DAC and amp. The Hifimediy offers a sound that is vivid and natural and dimensional in a way that the FiiO's DAC doesn't approach. I didn't expect this at all. Pianos sound like pianos, voices sound hauntingly natural, stringed instruments actually sound textured and vibrant. I may have to go on an ESS retreat weekend and learn their thetan kabbalist audio-gnostic alchemical secrets (or maybe just spend the money on claret and rioja, not quite decided...hmmm).
I appreciate that this is mere opinion. I have no measurements or credentials to support my opinion that the Hifimediy sounds better than another DAC using the same source files, amp and headphones. So I'll put it another way: even if you consider it sounds non-different to DACs A, B and C it is still remarkably cheap and so compatible as to be a safe buy for anyone regardless of hardware, and as such is better value than probably any other USB DAC you can find (I'm assuming you do want to ignore all the $10 colourful crap that resamples everything to 48KHz, adds a few crackles and burns out your USB ports).
If this isn't the ultimate value computer audio upgrade I'm not sure what would be. It is great value, plug and play on anything from Raspberry Pi to OS X to Windows, and sounds good enough it could make you lose sleep regretting the other stuff you bought.