It is plastic on the headband and around the outside of the earcups, with metal mesh covering the rest of the outside. Considering the original design was done in the late 90s, it does look a little cheaper finish than whats on the market today. The money, however, is all in the drivers and tuning. Personally I like my money spent on performance - and these deliver in spades.
My hd650 were incredibly durable. These hd6xx are from same factory in Ireland. Unless you sleep with these they will last you 10 plus years easily. And if something breaks you can get replacement part. Anyway I'm getting one. Amazing deal amazing headphone.
Going on 12 years with my hd600 with a lot of head time. No problems whatsoever. They are also very lightweight and, for me, a 10 on the comfort scale. Durability should not be a concern if you are using these at home as intended.
I think my HD650's are pretty durable. While plastic doesn't look as fancy, it actually makes for a pleasantly light headphone. and yes, you can see the plastic seams on an HD650 as well.