This was a really promising feature when it first came out, but became real apparently the idea was half started and so far seems it has already been abandoned. I purposely keep users away from this field to much, because of the missing support it has.

It needs support with Workflows, and business rules, and needs improvement with overall UI. A select all would be nice, and the ability to see the values selected better. Often the multi select collapses the selected values or cuts off the text on everything so you can't even see what is filled out. The multi select widget doesn't work nearly as well as you see it on other sites that have it.