The workflow UI is very cool but at least in the version I'm using it does not allow me to update a custom field on a transition. As far as I can tell I can only update "simple" issues (see screenshot). This feature would make it much simpler to add custom functionality to JIRA. It seems like quite a limitation because typically a workflow is updated to suit a company's process and in that scenario a custom field is likely to be used. I appreciate that I could write custom Java code for this but that's a level of effort I can't make at this point in time.