We will be in Umbria is May, and are looking into visiting the Frasassi caves, near Gubbio. Has anyone been there, and are they worth the side trip?
Short answer, yes.
When we were there, a long time ago, we were staying in a Le Marche town, so I can’t comment on doing the trip from Gubbio. There were English language tours at different times of the day. It would certainly behoove you to search out the times and make reservations, if that can be done. When we visited, the food offerings at the site looked pretty dismal.. We drove to the nearest town and had a very nice meal in a local restaurant.
I have visited Frasassi caves a couple of times and I think they are worth the trip. The last time I was there they had tours in Italian and English so you'll want to check the schedule for the language you're looking for.
Gubbio is a nice little medieval town and I enjoyed taking the funivia to the top.