It's extremely good, especially considering all I paid was £55. It's a sprung steel headband, and all steel connections. The grip is rather good, it may annoy some but I'm on the move with these a a lot and they won't be falling off. The only plastic is the decoration across the top of the band, either side of the foam protection underneath the band, and the ear cup covers. The plastic looks tough and durable. We will have to see about the paint finish (scratches/scrapes) but again it looks well lacquered and (hopefully) scratch resistant. The vent on the cup cover looks like it goes all the way in, obviously a port of sorts, but it doesn't leak sound and covering them makes no decernable difference to the sound. Unusual......
The only possible weak point I can see so far is where the cup swivel goes into the main headband. It appears to go into a hard plastic (sorry, there is more!) grommet, but this may just be how it appears. There are 2 tiny screws each side at the point also, meaning that the cups can be taken off, and rapairs, should they be necessary, can be undetaken.
If you covert a pair of these into planars then please let me know how it goes, I've just come over all giddy thinking about it, and mine will be the post to you rather sharpish