I have the M50x while my brother has the M50s and I've used the M50s enough to know that the durability with them is outstanding, I'm actually surprised my brother's cans still work good as new with how he treats cables and stuff.
Audio quality wise, they're pretty damn flat with a slightly more boomy but tight bass that isn't overpowering, and some very strong clear treble (considering that they are closed, where many do lack in treble)
The pads are super fluffy soft on my M50x and the pads on my brother's M50s are worn, but still very comfortable. My only gripe with these is that they don't have enough clamping force for me. I wear them at their smallest size and it's fine for height of the cups but there's just not enough clamping force when they're sized that far down. Very minor for me though, because when I'm out with them on, wearing a hat extends the size to the first/second notch and the clamping force is much more satisfying. With my Sennheiser HD558 I use two to four notches in size, so the M50s/M50x do pretty much have a larger fitting size and people with smaller heads will have trouble wearing them.
So far with my experience with the M50x after using the M50s on and off for about a year or so, I can say the detachable cable is VERY useful. You can use Sennheiser cables from their HD558/598 with the thin twist-to-lock 3.5mm style jack with no issues and no need for modding. (Which is super convenient for me whenever I want to use closed headphones while I'm at my PC, I switch out the cans rather than cables+unit. And this is super useful if you ever need to order a new cable for whatever reason, considering they give you a 1.5m Cable, a coiled cable, and a ~6ft+ straight cable with the M50x.
I've listened to a wide variety of music within my library from spoken word-like tracks to French Speedcore/Hypercore like stuff and Electionic styles sound very accurate with these, not much blending of sounds that I used to hear on headsets and cheap earbuds. And comparing to my HD558 they hold their ground firmly. Since taking the dampening tape out of my HD558 their mids and treble have opened up even more than they were before, without being over bearing, and the M50s/M50x don't keep up with the HD558 as much but are still very strong competition to them in clarity and luster.
The sound staging on these are also quite impressive for closed headphones, but given that they are designed for studio reference use that is expected to me. Still doesn't stand up to open designs when it comes to staging, but they surely make it up for it in their open-like range.
I'd recommend these to anyone who listen to anything that want good reliable closed headphones. So don't feel too hesitant, but if you can try out a pair first, that'd be your best bet. Most audio stores world-wide have them. (A local high fi audio store that mostly deals with super high end speakers only sells these, a few open AudioTechnica and many of Sennheisers mid-high end)