After listening to it all day, this is a very good headphone for sound quality, probably one of the better ones priced at its current listing. Its a very relaxed tuning that isnt harsh or fatiguing at all in any way, which is helped a lot and complimented by its bass. While being an on ear, the bass quality and quantity is wonderful. Its enjoyable and not bloated to my ears with a good heft to it. Mids are relaxed, with a bit of recession but its not particularly an issue that can ruin the experience for many people. Treble is also something I noticed that doesn't sparkle of cause sibilance, however I can definitely hear it to have a form of extension and not much of a fall off in that regard that really helps the tonality.
Staging is...ok? Its not going to beat an hd800 which is known for its wide sound, and I actually find the o2 to be a bit small sounding in depth. Not a fan of that imo, as with some of the timbre. I've felt there were times some of the instrumental separation was a bit closer than I would like. Some instrumentals didn't have that complete sense of punch that makes it feel satisfying and so, rather just like it is there. While I do enjoy a good punch to my guitars, this doesn't seem to really benefit in that regard and is relaxed. It's smooth, a little bit much at times yet there are also times it is very engaging. Some songs like Invisible by Duran Duran has never sounded so lively and fun! Feels like a new breath of fresh air for me. So while it may lack at times, there are also those magical moments.
Tracks that truly benefit from the o2 is honestly anything, and that is what I love about this. While some tracks can be lacking in a dynamic sense and the feeling I get from the overall sound, there are also times you just want to bop your head and get sucked into the music.
Comfort is up to the person, if you can handle on ears for long periods of time, its going to be really comfortable. However, some people haven't liked the current state of the padding, and I can see why. But, the ETA guys are actually working on that, to make sure they are improved and better which I greatly admire. Build is to be desired to the persons perspective as well. The o2 is certainly priced at a point where you wouldn't often see a premade headband and 3d fdm printing, but in this situation, I actually think the o2 has really clever design choices that justify that price tag! Its so small, robust and light that doesn't make me worried about it breaking any time soon. As a 3d printing enthusiast myself, this really makes me happy to see it being a used technique that standard production methods much like injection moulding and casting couldn't do ever.
It's not perfect in any way, but this headphone scratches an itch I find in headphones lack, which is passion. You do not see many headphone makers with that passion often. Only a few companies exist now that I can honestly say they're doing something new, and trying to make people understand that. As if it's an art form, and consider this a medium I am in love with.
Well done ETA.