AKG K-612 Pro is my second pair of high quality headphones. I had to ditch my Audio-Technica ATH-M50 in favor of this open-back model because of my chronic otitis (wearing closed-back M-50 during the day didn't help the treatment at all). I use headphones every day for work, making music, etc. And I must say my ears don't itch anywhere as much as before, so I highly recommend these headphones to anyone with ear problems.
K-612 sounds absolutely incredible. Just as good as M-50 in response, even in bass range, but with slightly different scene. Really no complaints here whatsoever.
The comfort level is pretty high, but I find the need to reposition the angle of the headband every now and then to maintain the comfort during long periods of use. The headband itself is just not that soft, so my skin doesn't like it. I think this may be more problematic for bald people, but I may be wrong.
I don't have any complaints quality-wise: everything is smooth, sturdy and nice to touch. I do have some concerns about the construction, though. Headband plastic bits look like they can break easily if dropped from a decent height, and I also don't like the usage of rubber bands for the auto-height adjustment, since they might stretch too much over time. But I din't see any comlaints about either of these nitpicks in other reviews, and unless any problems actually occur, it's a solid 5 star from me.
And last, but not least: anything you listen, no matter how quite the volume is, will be audiable for surrounding people. This is expected from open-back construsction, though.
All in all AKG K-612 Pro is a great choice for a fair price, if you absolutely need open-back construction headphones.