Pros: Very warm mids, great overall construction, fantastic design, descent isolation
Cons: Lacks detail, almost no soundstage, earpads very small, expensive
I've owned the Sennheiser Momentum for about a year now. Though it is aesthetically pleasing, and very sturdy overall, I can honestly say that it isn't worth the $300.
I enjoyed the mids, which are very warm and smooth. However, treble sounds very veiled. I enjoy headphones that roll off a bit on the treble, but the Momentums just take it too far, yielding very muddy high frequencies. Bass is adequate, even without an amp, but again, the lack of detail hits the bass hard. I believe these headphones simply lack detail in all frequencies, and while the mids benefit from that by acquiring the warmness that acoustically pleasing, the other frequencies fall behind quite a bit.
The soundstage is almost nonexistent. I have owned cheaper closed headphones and IEMs that produce a much better soundstage. The isolation is fairly good though, so I can't complain too much.
The leather earpads are nice and soft, however, they don't do much to help comfort for those of us who have larger ears. The Momentums simply are too darn small to feel comfortable, and I had to constantly take them off and have a break because my ears were hurting quite a bit. If you wear glasses, it's a lot worse. They are very light, however, so if you have small ears, I'm sure these will work fine for you. The clamping pressure can begin to hurt your scalp after long periods, but this isn't too much of a problem (or, more accurately, it is simply a problem for so many headphones that it is easy to overlook).
Also, the leather does wear down quite quickly.
Though these headphones don't need an amp, I did have to turn the volume up to 90% on my iPhone to get to a descent volume. I didn't notice much of an improvement in sound quality when I did use an amp, however, they do become significantly louder. Sadly, due to the fact that they are a very slow, closed headphone, turning the volume up a little will result in a further loss in quality.
As far as the design, these headphones do look very nice. They are sturdy, and I never found myself worried about breaking them. That alone might be enough to make this headphone worthwhile to some. However, again, I simply don't think it's worth the $300 it costs.
Overall, it's just too expensive to recommend. Again, I own other headphones that are cheaper, sound better, and are much more comfortable than the Momentums.