We decided against spray foam in our entire (large) house. Given our climate, the calculators put the cost/benefit payback at over 100 years. So far, I am happy with this choice except our playroom (over the garage with kneewalls and clipped corners on ceiling (so no cellulose possible) was significantly hotter than the rest of the upstairs this summer (unusually hot with temps over 100 for days on end).
This attic is separate from the rest of the attic (has its own access point). Does it make sense to foam only this area? Not sure of the exact square footage but it is over a 3 car garage.
Of course, lowering energy bills would be great but making the playroom more comfortable is also a priority. Rest of upstairs is very comfortable.