So - like a lot of people I was struggling with the Grease Cut in 2.78. I think I stumbled onto a two-part solution:

1. Stay in perspective view instead of ortho, like others have said. It takes a little getting used to but symmetrize helps a lot. 2. Always close your Grease Pencil path in a single stroke before clicking Grease Cut. Basically just draw a circle around the part that you want to REMOVE. This seems to make the object separation and Subtract feature work great. Great tutorial as always, Kent.