"Sí, la fiesta es divertida."
Translation:Yes, the party is fun.
This is why I don't care for the permanent/temporary way of teaching ser/estar. It is never really undersood correctly and often causes more confusion than necessary. Instead I suggest using the following two mnemonics:
SER: DOCTORE (D-date; definition, O -occupation, C-characteristics, T-time, O- origin, R-relationships, E-events
ESTAR: PLACE (P- position, L- location, A-action, C- condition, E-emotions.
In this case 'fun' is a characteristic of the party (so 'ser' is used)
Fun was originally a noun, with the adjective form being funny. For quite a while now (nearly a hundred years) people have used fun as an adjective to mean enjoyable and narrowed the meaning of funny to "inducing laughter".
Mark Twain used fun as an adjective. I think it's safe to use it the way we all use it without feeling guilty.