There are some really really really good holes on this course! The course is incredibly beautiful and serene! I can certainly see where some/many may say this is a "must play".
I rated it a 3.5 (grade of 88 = B+) which is the lower-end of what seems like a "must play" by most people's standards around here.
By comparison here is what I rated the top half of the 18 hole (and more) courses I have played in WI (limited (severely) to only the greater Milwaukee & Madison areas):
94 - Dretzka
93 - Brown Deer
91 - Hiestand
90 - Elver Park
89 - Vallarta-Ast/Token
88 - Fox River - Grey Fox
87 - Lustig
86 - Parkside
85 - Rock River
84 - Dretzka (temp layout for Worlds 2007)
83 - Village Park (Sussex)
82 - Dineen
Fox River loses some points with me in that it does not have anything too demanding in the shot-shaping department (mainly straight holes with minimum late turns/fades). Also, many of the short basket locations are too short to be fun/challenging for >300' throwers (which is fine for some, but not for me), and many of the longer basket locations make them tweener holes for 275-350' throwers (again, fine for short arms and also for long arms, but not for me.....nor for the majority of 900-975 rated players).