Berlin is an amazing addition to a Call of Cthulhu toolset. The book embraces this unique place and time with an immense number of historical roots, real places, and real events to make every terrifying story more real. I love that there is a balance between otherworldly and mundane horrors. Also any book that lets me roll for a random cabaret is perfection. The art, maps, and handouts are beautiful and very usable.
For a Keeper, the book works well for helping flesh out the region in your own game or as a set of scenarios with tons of support to make them come to life. The scenarios are gnarly and wonderful. Run them. ...But also check with your players to have the right safety tools available because they have some pretty raw elements (in the best way, but still).
For a Player, the book provides so many opportunities to build a rich character from Berlin or living in Berlin. You have details of all the regions, of real places, and of peoples of inhabit these places.