Worlds Without Number is a great system and I am not aware of any other book that is so helpful with worldbuilding. It is definitely worth picking up just for that even if you will be using a different rule system to play in. As for the rules, I really like the mesh of OSR spirit with more modern streamlining. The only thing that annoys me a bit is that the classes in the deluxe version are not simply called Beastmaster or Skinshifter but Llaigisian Beastmaster or Darian Skinshifter. That makes them harder to spot while scanning the contents as I will not play in the Latter Earth and hence not remember where they are from. Also the chapter is called "Arts of the Gyre" which is way more vague than it needed to be. But let us be honest, if this is the most annoying thing in a 400 page long book, that makes it a pretty good book!
[5 of 5 Stars!]