This conversation has been very helpful, as we are a new Marketo user and are running into the same issue. As another question pertaining to this topic, can we filter smart lists using fields within a custom object?
For example our custom object has product information and it keeps the history of everytime someone filled out a form to show interest in a specific product line. Each entry within that custom object contains fields like status of the product (active or inactive), and segment and subsegment information for each kind of product line.