Pros: SQ is just about perfect, build feel solid, paired with a good amp the sound is probably unbeatable.
Cons: sealed in battery, firmware is a little flaky.
I am not going to talk about the build quality, the size, shape, etc. Most things have been talked about to death by others. So, all I am going to do is add my own observations which would be best read in context with what other information is available out there.
You can plug a good set of cans into an AK120 and get an excellent clean signal. It sounds great. I then tried it with an Alo Audio RX amp and it sounded significantly better. Then I tried it with an RSA SR-71a and my jaw dropped. It suddenly went from being an excellent reproduction of an audio signal to being real music. Music as it should be. I close my eyes and am sitting in the studio while the album was being recorded.
Every breath, every rustle of background sound, the resonance of strings, the full spectrum from drums to soft or loud voices, to double basses, acoustic and electric guitars, everything rendered as though you're there.
No jitter, an amazing SN ratio, sound waves as clean and analogue as imaginable.
Yes, I would like slightly more user friendly firmware, better compatibility with my Macs, a user changeable battery, etc. I have most of my collection as CD quality ALACs and then a small but growing proportion of studio master quality files. With around 6000 tracks on the 3 * 64GB storage it takes about 5 minutes for the unit to scan through the library and compile it. It's not that long. Some people say it takes them that long for it to scan a single album... To be honest I don't understand that, perhaps it was on the 1.0 firmware, I purposely upgraded to 1.21 straight away without listening to 1.0 as others had said there was a change in SQ. I would rather just go to what I would be listening with rather than feel some real or imagined sense of loss after trying 1.0.
The bottom line is that if you have high end cans and want to pair this unit with an amp (I strongly recommend the RSA SR-71a which also, luckily has a similar width and length and so stacks well with the AK120) then for my money this is the best portable setup I had ever hoped for.
All in all, with good cans, the amp, a high quality interconnect and the AK120 the setup cost me around £3,000. For that money it is more than most people's home systems. For that money it should sound better than most people's home systems.
For the first time in years, possibly the first time ever I am actually able to just listen to the music without feeling cheated by the smudginess of jitter, the tin sound of poor treble, the over or under emphasis of muddy midtones, the trade off between either dynamic sound or clarity of sound. For the first time I just close my eyes and am swept away by awesome music.
I have been through dozens of portable units and combinations of units to try and get here. Now, with this setup, I am there.
If you just want the best sound quality possible in your pocket... BUY IT!
The battery life is okay lasting a good 10+ hours, probably near the published 14, but the inability to charge on the go or swap out the battery in my opinion is the largest failing for a portable unit. Oh well.