I've seen it about 7 times, and it is fabulous - don't know anyone who didn't love it. In all my visits I've never seen anyone in fancy dress, but people do indeed get up and dance to an Abba medley at the encore. The show is not the Abba story at all. It's an original story, set on a Greek island, which weaves together about 20 Abba hits. Many of those hits are introduced in corny but hilarious fashion, most notably Mamma Mia itself and Chiquitita. But the showstopper is an Abba song which was never a hit - it's a touching ballad called Slipping Through My Fingers. The show will make you cry as well as laugh, and it'll certainly make you want to dust off your old Abba records. I guarantee you'll have a blast.