They're incredible. I went through at least four bags in a week while I was in Japan.
Since you mentioned Amazon reviews, I went to look at them. The only negative reviews mention how theirs were rock-hard: I got one bag once here in America at an Asian supermarket where they were all quite hard. They weren't expired, they still had 6 months left according to the packaging, but they were way harder than they're supposed to be. I still ate them all because the taste is amazing. So I guess just squeeze one of them through the bag before you buy them. They should be roughly as soft as Starburst.
Speaking of Keion-related Candies: One of Yui's favorites, Kinoko no Yama/Matsutake no Yama(きのこの山/松竹の山) are also pretty good! The Kinoko ones are little mushroom-shaped candies with a biscuit stem and a chocolate cap. The Matsutake ones are the same idea, but with bamboo shoots instead of mushrooms. Yui mentions them in S2E04, when she passes a bag of them to the girls behind her on the Shinkansen. I'm quite sure Yui is seen eating Kinoko no Yama at some point, I want to say during the NatsuFesu episode, but I'm not sure.