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Castaway Club is the club for repeat cruisers on Disney. There is a separate check-in line at the port but it is not "express"; many times the regular check-in lines are shorter. I just pick the shortest line.
CC members do get an in-stateroom gift (it's been a black sport duffel bag and water bottle for a while now) and an invitation to a members-only reception where the captain and other staff members mingle with the guests. It lasts about 45 minutes and features appetizers and free drinks.