The short version: It's a solid case that will extend the life of your headphones. Ideal for mobile recording. Won't fit larger studio headphones.
I bought this case to protect a pair of Vic Firth SIH2 headphones when stored in a backpack to and from my rehearsal space, where I record drums and electric guitar. Previously I was using a pair of Superlux HD-681s, but stored in their soft bag inside the backpack the headband eventually perished. The Superlux weren't ideal for recording loud sources anyway, so switching to the Vic Firth I wanted to give them more protection, also given the cable is hardwired. The Vic Firth have a hybrid design much closer to ear defenders; the ear cups can be retracted right up inside the headband, and thanks to that they fit snugly inside the t.bone Headphone Case, with space for the cable to nestle beneath without placing any stress on the connection. The zipper seems durable, time will tell.
My AKG K240 MKIIs don't fit in the case, they're both too tall and too deep. I expect the same will go for many models of studio headphone. The material of the case has a little bit of give to it, it isn't entirely rigid, however I wouldn't recommend trying to squeeze in headphones with larger cups and risk breaking something. Thomann publishes the internal dimensions of the case, so just measure your headphones to be sure they'll fit when folded as small as they'll go.
Be kind to your ears, and be good to your headphones!