I'm have a little problem when I try the set some values before displaying the checkCombo.
I don't do anything really tricky, see below :
When the checkCombo, everything seems to be fine. The two selected entries are displayed in the combo, seperated by ','. But when I click to open it, no checkbox is checked.
var combo = Ext.create('Ext.ux.form.field.CheckCombo',
When I put a breakpoint on your "selectionChange" listener, I see a list (without the "All" entry) with my two values checked.
I don't really have time for now to investigate, so I report it now.
If I find something, I'll tell you.