It works well enough for me, more or less. Even dual dialog seems to work, but I can't guarantee the same results for you.
It only formats film-formatted FDX screenplays. I think Final Draft may do play and TV formatting too (?). If this is the case, it probably won't work with those.
As for Celtx files, if you (1) export your Celtx script to .html or (2) extract the .html file of your script out of the .celtx container file (use gunzip), it might work in this app. Or it may not. (You might also try viewing these files in a regular browser.)