The Holy Grail
All professional engineers will tell you "it's not about the gear, it's about the engineer." While I agree with that statement, I'm going to make an exception for this particular piece of equipment. In the last decade there have only been a few instances where I've noticed a big difference in my end product because of a piece of equipment. Immediately after receiving my Sennheiser HD800 headphones I noticed a big improvement in my mixing. What they did was show me in detail all the imperfections in what I thought was a deliverable mix.
The headphones reveal a relationship between the frequencies that no other listening medium has provided me before. The headphones reveal overbearing lows, "honky" mids, and ear piercing high frequencies. This "no BS" transparency allows you to make decisions in your mix that will ensure there is great translation between listening environments. As of now, I only have to make 1 trip for the infamous car test! In short, if your mixes sounding great on these, they'll generally sound great anywhere else you play them
I've always been a fan of Sennheiser's passion for audio with perfection being their end game. The HD800's are no exception to their flawless track record.
In short: If you're serious about mixing, invest in these amazing cans - you won't regret it!