My current case (Sonata 550) is about a year old, I would have kept using my old case (Sonata original), except I needed a new PSU anyway and buying a new case let me leave my old PC intact when I built this one up.
At the moment in use i've got
Antec Sonata 550 - current machine
Antec Sonata original - 4 years old, my old machine (now my mum's).
2x Globalwin 802's - both about 9 years old and still well worth hanging onto, one houses my dad's PC, the other a test machine.
Cheiftec full tower - about 8 years old, houses my main test rig.
I would probably still be using the 802's today if they had better cooling, as they are still pretty much the best cases i've ever had to work on/in, and that includes the original model Coolermaster ATCS 201 (I think it was), which was great but felt fairly flimsy compared to the 802's or the Sonata's.
It's odd, out of all the parts I've bought the cases seem to be the only ones that have actually stayed in (near) constant use over the years.