I replaced a RadComboBox (CheckBoxes="true") with a RadDropDownTree as the built-in treeview in RadDropDownTree much better (and easier) represents the data.
Specifically I miss the "ItemsCheckedString" and "AllItemsCheckedString" properties of the RadComboBox.Localization section when multiple checkboxes are selected (i.e. "3 campaigns selected")
<RadComboBox ... >
<Localization ItemsCheckedString="campaigns selected" AllItemsCheckedString="All campaigns selected" />
When multiple checkboxes are selected in RadDropDownTree, the DataTextField are simply listed in a separated string.