Updated review: 5/28/15
Build Quality: Great build quality. Overall solid professional looking open back headphone. Cable is strong and thick, non detachable cable isn’t/wasn't a big deal breaker for me, still compared to 7xx series, 612 uses lesser quality materials (plastic, leather headband) and can gradually loose the elasticity of the auto adjusting headband mechanism over time. Btw I have first batch of Austrian made 612pros, so I can't speak much for the build quality of Chinese built variants.
Comfort: I’d say pretty comfortable after few weeks. 612pro doesn't have much clamping force. Velour pads can be little hard at first, but after few weeks it will become soft, so have little patience in first few weeks.
Sound Quality:
612pro is one of the most neutral headphones in this price range imo. With right synergy 612pro can be a great all-rounder. Bass presentation is one of the star features in this impressive can, it can render deepest bass notes with exceptional texture, very rare for an open back headphone. Despite the ability to render the lowest notes still bass extension is very linear, so some might find bass is lacking little body. Impressive non-resessed neutral midrange is the other star feature of the 612pro. Neutrality of the midrange one of the main reasons why I prefer this to 7xx series, and many other cans. 612pro has a very good non-fatiguing treble presentation, I never found 612pro's treble bright, metallic or sibilant, but with some systems I found the treble can be dry, edgy due to the spike in frequency response at around 2khz which is very common with AKGs, so specially some female vocals can be fatiguing, dry due to this, so I believe that's the reason people recommending warmer amps to fix the synergy with AKGs.
612pro is not a basshead can by any means, but this is one of my top recommendations for EDM and many modern bass heavy genres, because this can render deepest low notes with exceptional texture. If you are used to heavy boomy, untextured bass presentations, 612pro will surely show you what you have been missing all these years, finally you can reinvent yourself as a real basshead who admires deep textured bass presentations. 612pro is surely is a fantastic option for vocals too. Vocals sounds very natural due to excellent neutrality of the mids, instrument timbre is very lifelike, very transparent, vocals can be upfront and can be very intimate sometimes. To me, this is a true all-rounder which can perform well in almost all the genres.
612pros soundstage is just perfect for my preference, it's not too huge or too airy, and imaging is exceptional for a medium sized soundstage, and can be a great gaming headphone too, if you are into competitive gaming.
But be warned, this is not an easy headphone you can drive without an amp, you must use a decent amp/dac to get he most out of this, I found 612pro is harder to drive than DT150, dt770.880.990 (250ohm variants) and many others, so be prepare to spend some money if you already don't have a decent amp, it can be a worthwhile investment in long run.
So, who might find 612pro interesting ?
If you used to brighter, leaner sound signatures like in q701, k702, still you might be looking for a bit more low end response, then you might find 612 more balanced and interesting. or if you are in to V shaped cans like dt990 you might find the non-recessed midrange of 612pro more intimate, or if you are in to boomy bass presentations, you might learn to appreciate the detailed bass presentation of the 612pro, or if you are just looking for a neutral open back option, AKG's 612pro is an excellent open back all-rounder.
