AKG K-371

What I immediately noticed after putting them on was that they might just be the most comfortable closed back pair of headphones I've ever worn. The earcups are simply flawless and while the padding on the headband looks pretty thin I don't have any complaints.
I was also really surprised by their built quality, having heard about issues with this model. I would go as far as to say that it is by far the best built AKG headphone currently in production, although not quite as well as the now discontinued K550 series. However just about everything looses to the k550 series when it comes to built quality. Overall they feel sturdy and use high quality materials with a great fit and finish. That being said the hinge definitely feels like an obvious weakpoint and I would have preferred it if they sacrificed a bit of portability (rotating the earcups inwards really doesn't make them much smaller) for durability, however only long term use can tell how well they'll hold up.

I was significantly less impressed with the sound. They are advertised as professional studio headphones and while they are useable for tracking thanks to their decent isolation, their sound signature makes them simply unusable for pretty much everything else you would do in a studio, as they might be the least neutral headphones I own.
The bass is quite elevated, however they somehow manage to be both bass heavy yet lacking punch in the low end. This leads to comparatively recessed mids making most instruments and vocals sound somewhat distant and muddy. This wouldn't be the end of the world, however it is exacerbated by a massive dip in the upper mids/low treble completely burying many harmonic frequencies in the mix, which causes the already quieter mids to sounds like they were recorded through a pillow. There is a slight emphasis on the mid treble but this does little to add detail or clarity and just emphasises cymbals.
Overall these are terrible for mixing purposes as the frequency response almost feels like it was purpose made to bury potential issues in the mix.
That being said they don't sound terrible when listening to music and they are very forgiving to bad mixes as they bury mistakes and are probably quite decent for bass heavy music like EDM perhaps. However with a lot of songs it can feel like you're being bombarded with bass instruments and cymbals through the headphones while other instruments and vocals are being played through a separate speaker on the other side of the room, which can make for a very straining and uncomfortable listening experience.
They do however have a great soundstage and god imaging for a closed back.

If you want extremely comfortable, well built headphones with good isolation and an ok sound for a reasonable price I can highly recommend them and I'll keep them for these purposes however I will definitely apply an EQ for certain music.
If you want headphones for professional work, you should stay away from these. The same probably goes for people who mainly value sound quality that being said audio is highly subjective and if you get the chance to test them you might end up liking the sound signature.
I would definitely consider them an overall step up from the similarly priced ATH M40x/m50x but I think one of the DT 770 models is still the way to go if you want a closed back in this price range and don't need a detachable cable.

I needed headphones for both mixing on the fly and for tracking. For that, they needed to not color the sound in a significant way, light, and closed. These succeed in all of those tasks.

Frequency-wise, to my ear there isn't any particular set of frequencies that stand out. No hissy highs or boomy (or absent) lows. Mid-range is controlled and tightly fit with the rest.

Easy to manage, quick to store away, and with different cables, it's easy to set them up in a way that fits the job at hand.
Field recording - good to carry around, easy to pack away, long cable if you need the distance.
Mixing - despite their being a closed design, they also feel like they breathe well, and their frequency response is smooth through and through.
Tracking - again, long cable to move around, and very light.

Overall, these are headphones that will fit more than one job description in a more than competent manner, and I would definitely recommend them.

Nearly 2 years of use review:
What I liked:
- Powerful bass, the 50 mm drivers pack a punch without blowing out the mids and being muddy
- Overall sound, these headphones have a nice natural sound with loads of detail which isn't lost to bass.
- Very responsive to EQ, these headphones can be easily tuned to whatever sound you like, analytical sounding to head vibrating bass.
- Cables, very nice selection of cables which are removable.
- Build quality, these headphones are very sturdy and they fold which is good for transport.

What I didn't like:
- Somewhat narrow soundstage, these headphones are closed back so the sound might appear to be quite closed in.
- Comfort, At first use the comfort wasn't great, the ear cups were shallow, the pads prevent use with glasses since losing the seal decimates the sound and the headphones aren't the most stable on the head. It can also get warm after a while.
Conclusion: Absolutely worth it. They are a great pair of headphones.