I'm working with a site that is currently making heavy use of comboboxes. In fact, every select in the whole application is automatically turned into a combobox on page load.
My first inclination is to take everything back to being a select and start comboboxing them on an as-needed basis.
But the question has arisen, when is a combobox really more appropriate than a select field?
(Combobox: a select-like dropdown of possible values, with a complimentary auto-complete style typing field at the top, which filters down the list as you type)