My buddy Bob Elisberg has written a good, in-depth article about several software packages via which one can write a screenplay. Knowing Bob, I assume he knows what he's talking about although the only one with which I have any real experience is Movie Magic Screenwriter. I've used it from its inception, back when it was called Script Thing, and I Beta-tested various versions and made suggestions. Its whole function when it outputs numbered dialogue scripts for animation recording was put in there because I suggested it and showed its inventor how to format such documents.
I've never found a way to make it work for me as a tool for writing comic book scripts, though some other writers like it fine for that. I still make mine in Microsoft Word with an array of macros that I wrote. But I do like Movie Magic Screenwriter tremendously for all other kinds of scripts.
That is the full extent of my opinion on such software. Anything else you want to know, listen to Bob.