The first time I saw the Domino XL I said to myself, "where was this thing when I was building doors in a millwork shop?"
We used loose-tenon joinery to connect stiles and rails, and since we didn't have a mortising machine, we used a lock mortiser instead (which we normally used to install mortise locks). The Domino machines (there are two of them) do more or less the same thing as a lock mortiser. But they do it faster, are easier to adjust, and you can buy loose tenons that perfectly fit the mortises they create. They can also cut mortises at various angles – though you probably wouldn't do that for a door.
You can compare the two methods of doing loose tenon joinery by reading this blog entry from Jesse Wright and this article Robby Myer wrote for This is Carpentry.
The video below was shot at JLC LIVE and contains a demonstration of the Domino XL.