What, academically, are you going to be doing at co-op? For us, it will be the "meat" of our week for History, Geography, Science, Art, and (foreign) language. So, it's worth it:) For my almost 8 year old, it would be... anyway... probably... (worth it) For hs aged students, it needs to either be a GREAT addition to what you're doing... (Like Science with labs) or... filling a great desire... like... say Art. (And serious art by then :)) A whole day, for just sampling subjects... may not be worth it by then.
For us, our curriculum is basically... "Open and Go" which is what we wanted. SO, we chose SOTW with cd, activity book, and student pages for history. (and we'll do one project each time.) So, teacher prep is minimal. I've already seen what parents who have to spend hours putting together curriculum...look like :) I wanted to keep it simple, but make the subject matter. :)