I was really excited to try out the Relationship custom field because I have been using the Task custom field to effectively create a look-up field. The best bit about the Relationship custom field is the ability to say which list contains valid tasks for the custom field. One example for us is a Clients list, that is then the lookup list for a Client custom field.
However, to make this feature really usable for us we'd need to be able to define a Relationship custom field at the workspace, space or folder level.