The original development of CSX was driven by some photographers and labs out in LA. One of their requirements was that they be able to process a large number of sheets in a single pass. So, with CSX, it is possible to point it at a directory tree with several thousand images in it and get a nice collection of contact sheets out the other end.
You also have the ability to specify a template file (a layered psd) into which the sheet is placed. This let's you automate the framing of sheets, adding backgrounds, etc...
For the captions, CSX adds the ability to use whatever fonts (and colors)
PS knows about rather than the limited set offered by CSII. You can also specify styles on both the captions and the thumbnails.
CSX also offers quite a bit more precision in the actual layout.
Check out the
pdf docs for more details.
It's still evolving, so if there is something essential that you think is missing, let me know.
-X