I have following suggestions regarding multicolumn combo. I'm sure you're already in process of implementing some of them.
- auto-complete functionality is vital for this control
- when user selects value (or writes down value, which is in the list), when dropdown is opened, displayed list (grid) should have row with that value selected. This behavior is in RadComboBox and it's very important in multi column combo too.
- there is BestFitColumns() function which adjusts width of columns but something that would adjust dropdown width according to width of columns is lacking and would help.
- there is one annoying thing - when you click on dropdown button of combo (that arrow which displays dropdown) when you are in process of validation of some control (you didn't validate control in Validating event handler by setting e.Cancel = true), multicolumn combo dropdown is still displayed. I'll submit this as a bug too, because it is causing me problems and is not expected behavior (nor standard .NET combobox control behaves this way).
I hope my input will help something and I'm looking forward to seeing this new control evolve.