I have been using these everyday for about 2 weeks now and here are my initial thoughts:
 

• The delivery of the musical soundstage isn't distracting in any way. Some people don't like this, as increased bass response or sharper highs are often valued - but I LOVE the flat, neutral sound of these cans. Music sounds as it should: detailed and harmonic. The cans don't get in the way and let the music shine through. Nice job Sennheiser.
• Some folks have commented on the bass response. Again, this comes down to personal preference. I don't particularly like increased bass response, but prefer a tighter sound in the bass. These cans deliver tight, controlled bass.
• I listen to all types of music: electronica is energetic and detailed, as are all of my jazz tracks. You can really make out the pluck of the bass and the grumble of the bari sax. I have REALLY enjoyed listening to Tchaikovsky as well, the soundstage is open and again - uncolored.
• Though I use a small amp at home, I've taken these out with my iPad and was pleasantly surprised. They are efficient enough to sound really great with portable devices.
• My only gripe about these cans is the amount of plastic used in construction. I realize that they are $300 retail, but I feel like Sennheiser could have made these a bit more beefy - more metal hinges, a metal headband, etc. The ear pads are very comfortable, and the cans are lightweight for all day use - I just find myself being extra careful.

 
Overall, I LOVE these cans. The neutral sound and wear comfort make them my favorite thus far. I see myself getting many years of happy listening out of them!
 
Cheers,
AO