I just got mine. The exterior frame is metal... Whomever said they were plastic is silly.
I'm not sure if it is plastic or metal, however, it has a nice feel and is well made. My first impression was that it is metal and I haven't thought about it until someone posted that it is plastic.
For example: the exterior of modern high-end Nikon SLR lenses feels like metal and is very well made but in fact it is made from some kind of "special NASA plastic".
I have the Zn for some time now and I love the sound. (Just to mention it, I'm not an expert at judging headphones and don't have much experience with real high end headphones, but I can say what I like for myself.)
They work great for electronic music and instruments (and voices) sound great as well, everything has a nice natural feel and sound.
I definitely prefer the Zn to the IE8 which I had before. The bass from the IE8 in comparison is just loud while the bass from the Zn is there and present as well with lots of detail but in a much more pleasing way without distracting from the rest (which was the case for me with the IE8). Furthermore the bass on the Zn goes quite some bit deeper than with the IE8.
The IE8 has a bigger soundstage with a wider and better separated placement of instruments but in comparison the IE8 sounds kind of artificial to me while the Zn has, as a already said, a very natural sound. The Zn has a more general soundstage with everything holding a little bit more together.
I really had some nice listening experiences with the Zn, my favorites:
Instrumental stuff by Chilly Gonzales: very impressive with LOTS of impact, I think the Zn plays this with ease.
"Tales of the Inexpressible" by Shpongle: totally got lost in the music and floated away.
Hudson Mohawke's "Butter": a joy fantastic with the Zn
no complaints about lacking highs here as well
Madeleine Peyroux and Nora Jones: nice and pleasing with the Zn
Today I'm traveling to Rome, lets see if the Zn will get another performance boost when listening to music with them in Italy