Pros: Balanced, refined, sturdy, classic design
Cons: not very "fast"
What? Nobody wrote a review of this gorgeous headphone yet? Maybe it´s because many people already own it and don´t talk about it - because once you do you won´t need another headphone. I use the HD-600 everyday and I did the same the six years before. I´ve never heard such a balanced headphone since and I think I never will. I´ve heard a HD-650 a few months ago but it didn´t make me happy, it sounded too dark and too warm. I missed the easy precision and crispness of my HD-600. Which is exactly what still baffles me: how was Sennheiser able to produce a headphone that is both detailed and laid back at the same time? It´s brilliant, but not too brilliant. It´s warm but not too warm. No, it´s a perfectly balanced headphone. Everything is there but nothing is too much. The soundstage is very natural, wide and deep.
I do a lot of private remastering and the HD-600 never failed me, they are analytical. I can hear errors easily and can correct them. Still, it´s not an overtly honest headphone. If it could talk it would say: "Observe these errors here and there, I´ll present them to you very pleasantly and neutral." That pretty much sums it up.
There are some minor obstacles: deep bass is a bit muted (from 20-70 Hz), depending on the source it may have some problems with differentiation. Another valid criticism is that it is not very fast - with the HD-600 you won´t exactly marvel at the punch, directness or dynamic of Punk or Hard Rock music. While this sounds a bit boring it isn´t - it just seems to present music as it is. They are more like passive "spectators", presenting music with a bit of distance. Another little downside is that it requires a good headphone amp, it needs a lot of power to sound good. If you don´t own something like that you´ll get an exaggerated upper bass - there goes your balanced sound.
If you think the sound to be a bit harsh you can buy the replacement cable that originally is intended for the HD-650 - with that you can add warmth at the cost of resolution. That depends on your taste of course.
All in all, I would buy it again anytime. But I won´t need to because it still sounds and looks wonderful. This headphone is the reason that I fell in love with Sennheiser, a love that is still growing.