^ Yeah, Iron Man 2 is one fairly "recent" example of the original U.S. slip being rare and in demand. There are plenty of others; there are lots of collectors of comic book-related stuff and ST/SW etc., they want "everything", the rarer the better. They shrinked the slips on the (re-release?)...
I like embossed/lenticular/etc. slips., but only when they "belong", like for a 3D BD, or like for e.g. Super 8 when it's almost a "necessary" part of the package design i.e. looks "wrong" without it (who recognises that BD without the slip pic?).
However, I can tell you one thing slips do...