My 840 are almost 3,5 years old (bought them late 2009) ,and the last 2 year I use them almost everyday.
I'm very careful with them,I've never dropped them down,and I never put them in a bag etc... When I use them,I adjust the headband to 9 position,and when I finish,I reajust it to minimum position and put them in the box.I don't have this white/grey cardbox thing in the box anymore,just the box alone.I use the carrying pouch in a way I found to make them stable in the box (I don't put them in the pouch,I just put the pouch in a place in the box,to make the headphones not moving much when I carry the box).
Mine are fine until now,no cracks on plastic,no disintagranting on the headband,and the earpads hold strong till now.But I do hear cracking sounds quite often,and the overall build quality combined with the comments here,make me feel a bit insecure about them.
Shure have to do a better job next time with build quality. My KRK KNS 6400 for example,feel cheaper but I'm 100% sure that they can get more rough use without problem,and they are almost 1/2 the price.