I have a 26" Marathon Plus on a commuter bike that has over 10,000 on it, and I can't see any wear.
I usually end up with tires all cut up and trashed before they wear out. The Ultremas and Duranos I love on my road bike usually die a premature death. (I love in te land of broken glass.) But I think that Marathon Plus will last forever.

I just got a new commuter that takes 700c and put 28 Schwalbe Marathon Supremes on it. I am impressed with the ride (if not the degree of mounting difficulty) and hope they are as tough as the 26" Plus. The price is a bit high, but darn they feel good. If the can survive the roads around here I'll be thrilled and consider it money well spent.