What are most people using for the front end GUI?

I think you are referring to the web service? (since the YAWL Editor is the only realistic option for creating YAWL workflows that I know of).

I have been investigating YAWL for some internal use, as well as a potential workflow tool for an upcoming project. For this I have been using just the default web service and forms as the interface where users will view and enact their tasks. It works well and requires no setup.