We also have a ZD331 w/ 72" deck. A great workhorse, indeed. I traded my ZD28 w/ 72" in for it. For lots of grass in a fairly flat area, it is super, but on hills, it's a bit unruly. It is a heavy machine, and once it is rolling, it likes to keep moving in the same direction, so going down a hill is a no-no if you need to stop, like coming to a pond!
I ended up buying a GR2110 just for cutting along the pond banks and on some other hilly areas. The GR2110 is awesome since it has 4WD for those tough areas.