Millsap has been a stud, so I am not sure Utah will give him up, but I do think this is an interesting trade. Although from the C's point of view, I think they would need Harris back as well (who Utah would gladly give up), since trading for Millsap is a win now deal, and you need to fill the PG hole if you are going to win now.
So, basically, it would be Rondo/JO for Millsap/Harris.
Utah saves money and balance the roster a bit more going forward, and the C's get a monster up front and a guy in Harris who is a lesser player than Rondo, but might fit in better with this core anyways.