Express check in worked great for us on the Golden Princess. In fact, each deck had 2 lines...plain and "express" check in.
We never even got our docs in advance....we live in Washington DC and our mail has been rather screwed up....in fact, to this day (we've been back from the cruise for 2 weeks now!) we still haven't gotten our docs!
So I did the information on the Princess web site and everything went very smoothly...they had everything ready to go for us, including the sign and sail cards, which had a green dot on the card! I think maybe that had something to do with it?? In any case, we had a great experience with express check in.