I have been using them for a year now. I also have a pair of Ath M40x. This pair is WAY more balanced. The m40x is closed and i use it for recording mostly, or listening to more popular music (while on train or bus) because it has more bass and hf. The DT-880 pro has incredibly neutral sound and boring (in a good way), perfect for mixing. Due to the fairly larger soundstage the sound doesnt become annoying at any point and it's fairly easier to close your eyes and imagine where the instruments are located in the mix (do that)
It also feels really sturdy and it is supposed to have all replaceable parts but nothing has broken so far. I have only taken them out of my room a couple times to listen to music at work though and i am pretty careful.
The only reason i put 4 stars in comfort is because even though the sound doesn't tire me at all, one of my ears touches the inside of the earcup and after an hour or two it kinda hurts and i have to take a break. BUT I HAVE GIGANTIC EARS. So unless you share my curse, you won't have any problems.
Also if you don't want a set Just for mixing, check out the dt990 which is supposed to be a kinda more "fun", v-shaped version of this one and it is way cheaper (and also prettier if you ask me)