I have just under 4,000 acres on mine and I think I have to replace the stationary knives this year.  Other than that I have just changed the oil in the gear boxes and set the knives up a few times.  The Roto discs still look really good.  So as for maintance I don't think is any worse than changing all the fails and D-rings on a stalk chopper every few years.

 FOR ME, the chopping cornhead pays for its self every year.  No extra tractor, No stalk chopper, No extra fuel, No extra hired help to drive it, No worrys...