We have bought them on a few cruises but the quality of the classes depends on the instructor. They have ranged from excellent, where the instructor took us under his wing and was very diligent, to an instructor who fell asleep in savasana after a ‘heavy’ night out. The worst was someone who couldn’t remember the (presumably, corporate) instructions and was following them on his phone! The other problem with them is that many classes are scheduled on port days during excursions so it can be difficult to attend. The best gym instructors reschedule for before or after but that’s rare.