We love celery- both the stalk and the leafy parts on top. When the humans cook with celery, we wheek excitedly because we know that means they'll give us some. Lola and I give it 5 stars.
As for Broccoli, we realized that he can't objectively rate food when he's eating with Lola and I. We've seen him eat celery in his cage and enjoy it, but you'd never know from the way he acts when the three of us share food. He pays no attention to the food on the plate. All he is interested in is us- puffing up his coat, butt swaggering, wheeking excitedly, running around us in circles. Why are you so excited all the time? The humans had you neutered!
Anyway, celery is a great treat for your guinea pig. 5/5 stars.