Combo Box was to replace reference and reference set selector.
The ability to handle that 1 - * relationship is great, however the one thing that is missing compared to the old way was the ability of using a table for the selection.
When you are setting many objects for the 1-* the text box gets really crowded with the bubbles, ideally the widget could allow for this dropdown view OR present a table beneath it with the selected associations.
I think this is a pretty common UX across the web and its a shame to not have it in Mendix.
At the minute we need to use a combination of the combo box & data grid 2 to get the effect.
Or maybe it could be a new core widget similar too this sort of UX