Been using VSX for a full year now. This has been said 1000 times. but I will say it again. If you are thinking of buying them, and you do not have the luxury to have a great room treatment, and multiple great monitors and headphones, Do not hesitate and just try them out. They are totally worth it.
Before this I had the Beyerdynamics DT1990 Pro, VSX just blew it out of the water. (Although not comfort wise obviously)
The biggest problem is that, no matter what expensive headphones or monitors you buy for mixing you will still want to make that frequency line straight as possible to even hear what are you doing, so you end up buying sonarworks or whatever. It just makes no sense.
VSX linear mode (the default mode) frequency line is as straight as it can basically be. AND YOU HEAR THE TRUTH. Only now you can make the right production and mixing decisions and stop wasting time.
One question I had before buying this - can they be used as the main headphones? Not only for mixing and production but for all the other things, even outside of DAW? YES. I sold every headphone I had, and I use VSX all the time. They are really great.
In DAW you just put the plugin in the master chain and that's it. You do not have to think about it at all. Outside of DAW you can still use them without any software! Although with VSX systemwide they sound better.
Comfort wise - I use them everyday. Even up to 10+ hours a day. I have no problem with comfort. They are not the most comfortable headphones as, for example, Beyerdynamics. But they still have a good build quality and you get used to them.
And as a bonus, for those who mix heavy music, like metal, which arguably is the hardest genre to mix. VSX is amazing. Now you can really hear the problematic clashing frequencies (and nasty high gain guitar noise) and finally can make a great sounding mix.