Quite good. I wouldn't use the term liquid gold in ear that Tyll did (I don't get that excited about audio...) but he has a point they really are very good. Like all other options there will always be some who says otherwise. You'll find a lot of people say the Westone (W40 for example) are better. I don't know I didn't try them yet.
 
I picked these because I find that while I enjoy the presentation of my Beyer t50ps they do tire me after a while and I suspect a few light headaches were caused by them as well. Looking at the measurement plots I can see the frequency response of the W40s has a familiar treble hump (just like most beyers). And I wanted something that's a bit more smoother. At the same time the bass on my Beyer while there was never great but I don't want anything bloomy. Again looking at the plots the 535s looks like they are pretty flat and the square response suggest that as well.
 
So I got them. And the first thing I did was run a bunch of different genre of tracks through them (Rock, pop, classical and Jazz). So here's what I found:
 
1) Bass is what I wanted, they are they and not the boosted bass you get in some phones. They are forward on tracks mixed that way and laid back on tracks where they should be
2) I can really hear classical with separation of the instruments! Nice
3) Vocals are great, mids are smooth and rich
4) Trebles are there when it should and not excessive nor tiring
5) Don't notice yet any graining to the sound as multiple BA IEMs can have that issue
6) The source is matters a lot. My nexus 4 now really does sound bad.

On clearence as there are T51p and dt1350s now so I went ahead and got these for use on the road and in the office. The other phones I own right now are the over ear momentums so this review is in some ways a comparison. 
 
I was a bit apprehensive due to the negative reviews here on head-fi from some owners. But I was somewhat pleasently surprised when I received them.
 
Build, looks and comfort:
 
The momentums win this one hands down. The T50ps has more of a raw industrial look to them, while the Momentums looks like (to me anyhow) a set of classy headphones. This is sitting on the desk, wearing them I have to say the T50p looks better while being worn. Comfort wise the momentums wins, they are so light and well designed they almost disappears when you put them on. While the T50p are not uncomfortable as such, they feel fiddly to adjust, has more clamp pressure and feels less secure on your head.
 
Materials wise I think Momentum are better, the fact that the T50p uses a set of bare rivets for the ear cup pivot make them look a bit cheap.
 
Design wise you have to wonder who approved a the T50p where adjusting the head band length will almost always (on a guy with shore hair) result in painful caught hairs? 
 
Audio:
 
Well as a newbie I don't know how much my impression would mean to the old salts around here. Anyhow, I found them to be quite good. Yes, the Momentum has better sub bass and better bass in general. But the T50p are not bass-less as some might lead you to believe. And in some tracks I found more details in the bass in the T50p than the momentum.
 
Overall the Momentums are better overall with a more "effortless" presentation. But the T50p does some sounds better in the mids and treble. 
 
For what's it's worth I never encountered that tricky positioning issue that maybe only the early production models experienced. The only way I can reproduce the nasal vocal description was to move them into a postion where it is very clear to me they weren't sealing. In fact they sounds perfectly good the first them I put them on my head without fiddly adjustments.