When I have enabled FixedHeader with {"left":true}, the column headers don't change width when the table is resized. You can see the problem here: http://live.datatables.net/akedig if you load (looks fine) then resize the window so the table and column widths change.
That example is just the default example + FixedHeader({"left":true}) + remove the "container" div (so the table is not fixed width) + <thead style="background-color:white">.
That example is just the default example + FixedHeader({"left":true}) + remove the "container" div (so the table is not fixed width) + <thead style="background-color:white">.