I finished my progress book for Madoka Kaname yesterday and while I was getting critique, one of international cosplay friends commented that they had never seen one before. In the UK they’re quite common now to go with competitions; most major competitions (including both European Cosplay Gathering and World Cosplay Summit qualifiers) now require progress books, and many local competitions encourage you to submit one too.
I think they’re a great little momento for cosplayers to make and keep, especially for bigger costume projects! Most events will give you your books back so it’s a nice keepsake and chronicle of how you made your costume and they’re great if you want to share them and show your construction to others.
My first progress book was for Eiko Carol, which was very well received by the judges. It was a lot of fun designing it to match my costume.