I'll start with the good.
As has been said by many, the sound signature of these headphones is non-fatiguing and they can be used for many hours continuously. Voices sound amazing and contain none of the annoying sibilance. Clamp force is a bit high, but loosens afterwards and they become really comfortable.
Now, for the bad.
The Sennheiser veil is present. Only a very minor gripe since voices are still amazing and the roll off on the highs is what gives these headphones the non-fatiguing sound signature, so it's a worthwhile trade off (a 1db boost at 2.5khz and 8khz basically solves this anyway).
However, the main problem is the midbass; it's boosted enough to be distracting, otherwise they would be my favorite headphone. It is especially distracting when there is heavy midbass and it starts making it harder for the voices to shine through (thankfully it does not distort or muffle the voices). I assume it is there to compensate for the roll off below 80hz, but the result is simply an annoying hump. I guess this works for the majority of people since this is where these headphones get their "thick" sound. However, for me, it is unacceptable way of compensating for not having a flat to 30hz bass response. However, a bit more EQ to even it out and they become passable.
That's it for the review, there's not much else to say since these things have been reviewed endlessly by many others. Just figured I'd my review of how I was a bit disappointed by them (granted, I had EXTREMELY high expectations from them given the reputation the HD line carries). The rest is not directly related to them, but just my experience with them that might serve as comparison.
I had wanted the HD headphones for about 4 years now, but 3 years ago, I ended up getting the Beyerdynamic DT880's instead. Why? because I got them for $117 and they had good reviews as well. To be curt, I was disappointed with them, too.
Basically I just wanted a nice pair of headphones to see what the hype was about and have decent sound when it is not possible to listen to my house system since that one is not portable nor quiet, but headphones are (and yes, my house system does sound better, so they were never meant to replace it)
The DT880's weren't perfect for my purpose since at around 8-10khz they sound harsh, and it becomes extremely annoying on some recordings in which they thought that it was a good idea to go crazy on the high hats and boost them right at that frequency. However, they proved very useful as a basis for comparison, while a simple 6db cut at the offending frequency spike turns them into an extremely clear, and revealing heaphone with a bass curve that is pretty much perfect (albeit still with a slight roll off).
Of course, I still wanted the HD's since I thought I had made the wrong purchase and the HD's would be perfect. So when these went on sale, I finally decided it was time for my next pair of headphones.
When I finally received them and got to try (and compare) them, I was pleasantly surprised how similar the sound signature was to my home system. Sadly, one of the main differences was the hump in the midbass.
Since I now had two headphones I decided to buy a tube amp (again, mostly to see what the hype was about) and see if hopefully the HD's became even better. As to not make this any longer, I'll just summarize the rest: 1) didn't do much for the HD's 2) ironically, it improved the DT's since it mellowed out the offending high frequencies 3) I liked it enough that it is now basically being used as preamp in the line for my main system (not life changing difference, mind you, but I figured that it would be a waste to only use with my headphones which are unused if I can listen to my main system which is about 95% of the time)