These headphones sound really good, better than any other headphones that you see in Apple store (even the new B&O H6).
Like we all expect from Sennheiser, this headphone has a very good sound quality. You will notice right the way when you first listen to it that the sound is very clear. The highs and mids are very good. The sound is very SMOOTH even at high pitch. Don't believe me? Try to listen some songs by Adele at high volume level, headphones like Bose QC15 will kill your eardrums but the Momentum will still keep the vocal very smooth. Some people may feel that the Momentum lacks of bass, but I personally think that the bass is just right, unlike the artificially boosted bass of the Beats by Dre or a almost complete lack of bass of B&O H6. The sound stage is OK/good for a closed back headphone.
In my opinion, the Momentum is made for various kinds of music, except those with heavy bass, such as rap, techno, etc. If you prefer headphones with more bass, go for Beats by Dre. They really boom your ears like a subwoofer.
Compare this to the new Beats by Dre Studio, the Momentum has a lot more to offer at the same price range. Better details, sound stage, mids and highs, and most importantly, it has a lot more natural sound. But I do find the Beats to be more comfortable to wear due to its larger ear cups.
Compare to B&W P5 and P3, the Momentum has less bass but more clarity.
Compare to Bose QC15, the Momentum has a warmer and smoother sound but less comfortable to wear.
Compare to B&O H6, the H6 has a flat sound and almost no bass but more comfortable to wear.
The sound of the momentum is SMOOTH (I know I said that, but I just wanted to remind you)!
The built quality is good, except for the plastic housing which in my opinion doesn't fit with the overall quality since the rest of the headphone is covered in leather and metal.
The headphone can be powered from your mobile device since its impedance is very low (18 ohms). But I really do suggest using a laptop or a portable amp to get the best out of it. It doesn't make any difference if you just listen for fun, but if you want to sit down at a quite place and listen to your favorite music, you need a good output device. I personally use a portable amplifier with the Momentum.
The ear cups are small for some people.
Finally, the brown leather pads are a lot darker than those in the photos. Also, the leather is a little dry. Hopefully when you'll get yours, you won't have this issue.