tl;dr: If you're looking for open headphones for $200 or less, buy these. End of story.

Sound:
The word immaculate comes to mind. I've previously used the akg k7XX extensively which are reference quality headphones and work amazingly but I found they sometimes added a bit of a harshness to the audio. If I had to express how it sounds with words I would describe it as a "surgical precision" like a scalpel making an exact cut. The HiFiMan HE-4XX are similar in that I in no way doubt the audio they provided is exactly the way it was intended to sound but I never noticed any harshness, perhaps it is because of the planar magnetic drivers but the feel I get from these is an absolutely flawless listening experience that is still always fun without coloring the sound.

I've tested these with an "all purpose" use in mind. Everything from youtube vids, music from a variety of genres with focuses varying from metal to edm to acoustics to vocals, podcasts, and of course gaming.

Comfort:
The first thought I had upon putting these on my head was that this must be what it is like to be a dog and live your life with ears made of velvet because holy balls are these focus-a pads some of the most comfortable things I've ever had on my ears. And the padding under that velvet (Disclaimer: material is actually velour) is freaking perfect in terms of firmness and thickness. The second thought I had was that this must be what (Game of thrones tv spoilers) Oberyn Martell felt like because the clamping pressure on these puppies out of the box is like the Mountain is trying to crush my skull. (Disclaimer: I have a large skull so I get hit worse than most people by excessive clamping pressure.) 24 hours of use and some bending later and they have broken in nicely and have a completely comfortable amount of pressure. The headband is covered in leather which is also extremely comfortable. For reference in addition to the Akg K7xx I've also owned Mr. Speakers Mad Dog Pros and I sewed an extra padding strip on both of those headbands but felt no need to add anything extra to these.

Aesthetics:
Leather, metal, velour, black, and ...midnight blue? Okay I guess I should be glad because the massdrop picture looked like this awful purple was used to accent these headphones but why not just go black on black instead of black and a shade of blue that is one notch away from black? It is a small nitpick because 99% of people seeing these would probably not notice or even think about it but I'm ocd and batman is my god damn spirit animal so give me the darkness my inner edgy emo craves. I like my headphones black just like my metal.

Build quality:
Fantastic. Spring steel headband wrapped in leather, metal pivots (blackened to match the aesthetic), velour pads with perfect foam, easy to drive planar magnetic drivers, and an easily replaceable cable that terminates in 2 3.5mm plugs and not some god awful proprietary alien looking connector!

Impedance:
That's correct. My Galaxy S7 edge powers these things to "seriously uncomfortably loud" at max power which is a step or two under "screaming" on my scale. My comfortable listening volume is a notch under the 3/4ths mark (So about 65-70% volume). Ladies and gentlemen, these are planar magnetics you can take with you on the go! Why you would take open backed full sized headphones out instead of sound isolating iems remains a mystery to me but at the very least you can use these at home without an amp if you so choose.

Price:
At a $170 price tag I honestly can't think of any other headphone on the market I would want to own more. Even the Sennheiser HD 6XX costs $30 more and the HiFiMan HE-400i I'm told this model is based on costs $50 more and has a seriously inferior headband. Anything else I would consider buying over this is in the $500+ range.

Cons:
The stock pads get warm and in the summer I'm certain these are going to be some real ear sweaters. Luckily they are easily swapped.
The adjustment slider doesn't click into place. It has a firm grip and I've done the headshake test and had 0 slippage but I thought I should mention that is the only compromise I can notice that has been made on the massdrop version of the product. (But the headband is massively improved so I'd call that a more than fair trade for $50 off, also 3.5mm jacks are used on the massdrop version which is a huge improvement over the stock HiFiMan 400i model's weird prong things)

Overall:
I give these a 9.5/10 and are making these my recommended buying headphones. (Combine these with a Schiit Fulla 2 and you've got the best bang for your buck budget setup on the market. Or a Mayflower Arc if you want to run a mod mic on these suckers.(The sides don't have a good attachment point unless you want to go high and use the adjustment clamp for the sliders but the bottom is thick enough))