I have never seen anyone use it properly - in one usability test with experienced web authors and a couple of developers, not one person could successfully select more than one keyword from a long list.

The multi-select combobox seems to me to be one of the worst controls ever, particularly for a new user and especially for a long(ish) list. Which items are selected is nearly impossible to tell. The idea of Contol-Clicking to select additional items is not easily explained to the new user. My suggestion is the alternative of many checkboxes, all with the same name or ID, and a little extra code on the loading of the form to check those boxes that need to be.

In my opinion, the multiple select box is the worst HTML form component to use in any case. No other component forces the user to use the keyboard and the mouse simultaneously. I don’t like to assume the user even knows how to use the shift and control buttons with the select box. I would use it only as a last resort.