Other than the sound quality, here's what else I like
The foldable design is quite good. But I'm not sure how that will fair over time with someone like me, who is quite clumsy and unintentionally heavy handed. Time will tell.
Noise containment is really good. The headphones are like bins, they have a very deep, thick and well fitted ear pad cushions that really do a good job of cancelling out a lot of outside noise, and keeping the sound within where it should be without much leakage. This changes though once they are taken off if the sound from the headphones is still running / playing. Then they are just as loud as any other pair of headphones in terms of noise leakage.
What's Just OK
The microphone. It's just okay. If you're a gamer, and you want to talk to people online while playing a game, I wouldn't really recommend these headphones for that. It's not terrible, but there are much better gaming headphones available that offer much higher, clearer sound mic quality when communicating online through gaming servers. Then again, the majority of purchasers don't buy these headphones for the mic, so most people will probably never use it anyway.
What I don't like
The headphone jack. As in the actual part that goes into the external device. I say this because the Sennheiser HD400S also have a jack connection that attaches to the headphone cup itself [see the pic below], albeit a smaller one than the usual 3.5mm. See the first picture below. This smaller jack is used for attaching the connecting wires / cable to the headphones themselves, and it can be replaced but the design is not as generic as the average jack connection. It has a little groove cut out on the side that twists and locks into place.
So some people are not aware that the smaller jack that attaches to the actual headphone cup needs to be inserted and twisted / locked into place. If it isn't locked in, the headphones will only work through one ear, and some new users may think they are broken. But its just the design. This isn't good though, to have a specialised jack like this.
I would imagine a replacement would have to either be purchased directly from Sennheiser themselves, or you may be able to find someone selling these jacks with a side groove. I don't know how popular this design is. The headphones really should have been hard wired to the cups to avoid annoyance and confusion.
I know someone who recently sold a pair of these headphones on eBay, and this exact issue popped up. Headphones were sold and all seemed well. Then, the buyer contacted the seller and said they only worked in one ear. The seller explained that it was just the design and the headphone worked perfectly, but the buyer didn't want to listen, and the explanations what needed to be done to lock them into place fell on deaf ears. Pardon the pun.
He had become concerned because they appeared to only work through one ear. That spooked him and he didn't want the headphones anymore. All he really had to do was twist / lock the smaller jack into place and they would have been perfect. But no, he wouldn't or couldn't seem to figure it out despite the seller even sending him a review video which showed the exact silly "feature" being talked about and demonstrated.
The headphones were sent back and the buyer received a refund. The buyer later apologised, acknowledging that he was wrong about them being faulty and it was the design.
But back to the normal 3.5mm jack that attaches to an external device [last picture below]. For most things the headphone jack seems to work fine, but with a smartphone it does not appear to fit well. Maybe its just the smartphone I was trying to use them with, I don't know.
Not the best fit when used in a smartphone
