Just got my pair today. Wow, this is really worth its price. I think it's very comparable to headphones up to the $300 price range. I'm surprised by how much of an upgrade to my ATH M50X considering it's only $20 more expensive.
I'm a college student, and I use it while in the dorm or library. The sound leak is minimal that I can push the volume to a reasonable degree without having friends around me complain. They might hear something, but it's quiet enough that people don't really bother. Especially if you get the volume down, and its sound leak really blends in to the ambient noise unless you get your ears a feet within the headphone grills.
Since I'm nearly always on the go, I didn't really invest in a dac/amp to lug around all day. This pair of pure awesomeness is easy to drive, and it can get uncomfortably loud when cranking my volume to max on my smartphone. To be honest, even though it's 4 times more resistant than my M50X, but it's only about 30% to 50% harder to drive to the same volume. Sound on phones and laptops aren't bad. They don't get muffled up and quiet like headphones with higher impedance. The sound straight from my phone is still very enjoyable. I don't have a dac/amp around, so I can't judge how much potential the headphones have left, but they're definitely better performers than my M50X on smartphones.
Compared to the M50X, it's got a wider sound stage, not huge, but decently so that I feel awkward going back to the M50X when listening to some more demanding tracks like James Blunt's "You're Beautiful". In most songs where the left and right channel don't differ that much, it's a similar experience, and I have to REALLY pay attention to identify the difference. I heard the HD58x's sound stage is smaller than its older brothers, but I'm OK with that since it's only $150 (only half the price than HD600, the previously cheapest headphone in the lineup), and it's an upgrade to my M50X. If you only got closed backs before, I definitely recommend these for the price. The only thing you lose is the noise isolation, which if you don't take the bus/train/plane a lot, it's going to be sufficient. Especially if you already have headphones or iems for that purpose.
Then let's talk about sound (on my smartphone). Simply put, it's great. It's a steal for the price. It doesn't look premium like the other HD 6's, but it doesn't slouch too much when listening to them. The sound is pretty neutral, not too much bass, not too much treble. The sound lines up well with the M50X, which is a pretty flat monitor, but maintains the Sennheiser richness that its brothers are renowned for. It's got a good separation, allowing me to recognize the different instruments in an orchestra even when most of the orchestra is playing together. The sound is fun to listen to, and not fatiguing at all. The highs are controlled, but brilliant. Soprano voices are absolutely beautiful, and energetic. "Der Holle Rache" is a piece that reaches High F for the Soprano, and these headphones rendered them with ease and elegance. The beauty of the highs led me to discover that the Renee Doria version of "Les Oiseaux Dans La Charmille" on Spotify has ugly crackling sounds, which I didn't notice before. It's such stark contrast to the other songs that it becomes unbearable, something that never happens with my M50X or Shure SE215. Switching tracks to Aya Makino's version, the sound is great. It even manages to make that notorious prolonged High Eb at the end sound tamed. Lows are well controlled and kind of "bouncy" if you know what I mean. In songs, the drum rolls come off nicely, not overpowering, but still clearly discernible. They are much less muddier than those of my M50X. Mids are nice, not really a lot of comments on the mids. They sound rich, but not overly dark. The cello sounds are especially beautiful in this range. Put up some of Yo-yo Ma's solos and you'll understand what I mean. Great sounds overall, but since I don't have dacs or amps lying around, I won't say how far these will take you. It sure isn't the best sounding headphones, but for this price, and its ability to be used on portable devices, it's definitely worth a try.
Finally, comfort. A pair of great-sounding headphones is worth nothing if you hate the feeling of them on your head. The HD58x is exceptionally comfortable. I wear glasses, which usually limits my time with the M50X, but not with the HD58x. They are comfortable as hell. I have been listening to them for nearly 6 hours straight right now, and I'm still willing to do it for another few hours. The cushioning are soft enough that I don't even feel my glasses, and the clamp is just right for me. If they do feel a little tight, you can always stretch the headband to make them a little looser. The adjustment for head size on my unit is pretty tight, and I need to use a concerning amount of force to adjust it, but it's not meant to be a "take it with you" kind of headphone, so I don't need to do this constantly.
Overall, it's a steal at this price. For those who don't have too much experience with headphones, this is surely one of the best entry-level headphones money can buy. It's got a long cable, but you'll get used to it eventually. For those who already own some nice headphones, this will be a great addition to your collection, especially at this price. It's now going to replace my M50X as my daily driver.