This SEXP is kind of a mess because it requires you to manually add, and in a quite disturbing way to be honest, all information related to the messages you want to be displayed on screen. It takes a lot of time, much more than what I'd expect from a SEXP that is supposed to make things easier. As Battuta said, it's not as good as using a chain of events and send-message. I admit it's the SEXP I use, however, and I chained up to 30 messages with it.
If I recall correctly, unless things have changed in the past few years, there's also a limit to the maximum number of messages which can be used with this SEXP. (Correct me if I'm wrong, please).
It would be much more handier if used on a completely different interface that would allow you to select a number of messages in the box and get them "transferred" instantaneously to the SEXP. The procedure I dream of would be something like this: select the messages from the box, then get them to appear immediately in the SEXP in the same order they show up in the box itself. At that point, you'd simply have to select their recipients, intervals in milliseconds, and priorities. It'd be much, much faster.