I've only had mine for 4 days, but no case here.
No dings or scratches yet.
I'm always reluctant to use cases on my devices, but I have one of those matte spigen one's coming tomorrow. It's gonna be a pain having to take off the case every time I want to charge though...
I currently have a Galaxy S5 and I figure the back will be cheap to replace but I'm disappointed with the camera performance and now considering the Z2 but worry about its durability.
It's like any phone pretty much... Don't drop it and you'll be fine. Both glass panels are very durable and don't scratch easy. The aluminum frame is just as tough as well.
Accessories are hard to come by as expected. I bought an expensive Pdair slip case, but prefer my six dollar Galaxy Note 3 slip case because of the magnetic flap top. I worry about dropping the Z2, so I never handle it without a wrist lanyard just like when I'm using my RX100 camera.
I'm using mine naked. My I only gripe with the phone would be that all though it has an aluminium outer edge the outer edges of the aluminium are actually plastic. I have chipped into mine a little as it fell down the side of the sofa and hit something metal I guess. Only a small mark like but if dropped I imagine the plastic bit would take a good hit.
To put a case on the Z2 is wrong infact you buy a phone for many reasons but aesthetics is a big one.
So to say that you like the design and then slap on a case which changes that. In this case the fine build of alliminium and glass and then to put it in a plastic case to me doesn't make any sense.
Yes I know the benefits of a protective case. But they all defeat the purpose IMO.