My wife and I just returned from a 13 day cruise from Rome to Barcelona; crossing through Greece's islands.
In sum, Celebrity get a 10 out of 10 for their hotel services (stateroom, cleanliness, customer service, food). The ship is kept incredibly clean and updated. Evidence of this is how you will see the crew constantly washing the decks and other assets with fresh water constantly.
Another aspect worthwhile mentioning is the hygiene practices they live and die by. As an example, throughout the day (literally) there are crew members disinfecting handrails and "Pureling" every single guest.